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Cellular Modem Setup Guide - SIMCOM SIM7600G-H on Raspberry Pi 4

Overview

This guide covers the complete cellular connection setup for your Raspberry Pi with SIMCOM SIM7600G-H modem. Three complementary scripts work together to provide reliable, automatic connection management.

Problem: Dynamic IP Addresses After Reboot

Cellular carriers use DHCP-like assignment pools. Each time you reboot, the modem gets a different IP address from the carrier's pool. Hardcoded addresses become invalid, breaking connectivity.

Why This Happens

  1. Carrier assigns addresses dynamically: Unlike fixed broadband, cellular networks assign temporary IPs from a pool
  2. Each connection gets new addresses: When you reconnect (reboot), you get different addresses
  3. Manual configuration breaks: Hardcoded IPs from a previous session no longer work

Solution: Three-Script System

The solution uses three complementary scripts:

  1. connect-cellular-robust.sh (Recommended for production)

    • Handles modem state management
    • Disconnects existing bearers before creating new ones
    • Delegates DNS/routes to helper script
    • Works reliably after each reboot

  2. connect-cellular-dynamic.sh (For troubleshooting)

    • Manually extracts and configures IP addresses
    • Shows detailed configuration information
    • Useful for understanding what the modem assigns

  3. auto-recover.sh (For high availability)

    • Continuous monitoring daemon (every 30 seconds)
    • Automatically detects and recovers from connection loss
    • Checks routes, DNS, IP connectivity, and DNS resolution

How They Work Together

The robust script handles initial connection:

  1. Connects to the modem using ModemManager
  2. Queries the modem for the assigned IP configuration
  3. Extracts the actual addresses from mmcli -b 1 output
  4. Configures the interface with the correct addresses
  5. Works reliably after each reboot because it uses whatever addresses the carrier assigns

How It Works

# Step 1: Enable modem and create bearer
mmcli -m 0 --enable
mmcli -m 0 --create-bearer="apn=ereseller,ip-type=ipv4v6"

# Step 2: Connect the bearer
mmcli -b 1 --connect

# Step 3: Query modem for assigned configuration
mmcli -b 1
# Output shows:
#   IPv4 configuration |         method: static
#                      |        address: 10.19.145.184
#                      |         prefix: 28
#                      |        gateway: 10.19.145.185
#                      |            dns: 172.26.38.2

# Step 4: Extract and use the addresses
IPV4_ADDRESS="10.19.145.184"
IPV4_PREFIX="28"
GATEWAY="10.19.145.185"
DNS="172.26.38.2"

# Step 5: Configure interface with extracted values
ip addr add 10.19.145.184/28 dev wwan0
ip route add default via 10.19.145.185 dev wwan0

Quick Start

Step 1: Make scripts executable

# If scripts are in current directory
chmod +x ./connect-cellular-robust.sh
chmod +x ./connect-cellular-dynamic.sh
chmod +x ./auto-recover.sh
chmod +x ./setup-dns-routes.sh

# Or if installed in /opt/cellular
chmod +x /opt/cellular/*.sh

Step 2: Connect the modem (Production)

For production use, run the robust script:

# If in current directory
sudo ./connect-cellular-robust.sh

# Or if installed in /opt/cellular
sudo /opt/cellular/connect-cellular-robust.sh

Step 2 (Alternative): Connect the modem (Troubleshooting)

For troubleshooting and detailed inspection, run the dynamic script:

# If in current directory
sudo ./connect-cellular-dynamic.sh

# Or if installed in /opt/cellular
sudo /opt/cellular/connect-cellular-dynamic.sh

Expected Output

[INFO] Starting cellular connection setup...
[INFO] Step 1: Enabling modem 0...
[INFO] Modem enabled successfully
[INFO] Step 2: Creating bearer with APN=ereseller, IP-Type=ipv4v6...
[INFO] Bearer created successfully
[INFO] Step 3: Connecting bearer 1...
[INFO] Bearer connected successfully
[INFO] Step 4: Bringing up interface wwan0...
[INFO] Interface brought up
[INFO] Step 5: Retrieving IP configuration from modem...
[INFO] Retrieved configuration:
[INFO]   IPv4 Address: 10.19.145.184
[INFO]   Prefix: 28
[INFO]   Gateway: 10.19.145.185
[INFO]   DNS: 172.26.38.2
[INFO] Step 6: Configuring interface with retrieved addresses...
[INFO] IP address configured: 10.19.145.184/28
[INFO] Default route added: via 10.19.145.185
[INFO] Step 7: Setting MTU to 1430...
[INFO] MTU set to 1430
[INFO] Step 8: Configuring DNS...
[INFO] DNS configured: 172.26.38.2
[INFO] Step 9: Verifying connectivity...
[INFO] Interface configured successfully
[INFO] Connectivity test PASSED - can reach 8.8.8.8

==========================================
Cellular connection established!
==========================================
Interface: wwan0
IP Address: 10.19.145.184/28
Gateway: 10.19.145.185
DNS: 172.26.38.2
MTU: 1430
==========================================

Step 3: Start Auto-Recovery Daemon (Recommended)

For high availability, start the auto-recovery daemon:

# If in current directory
sudo nohup ./auto-recover.sh 30 > ./auto-recover.log 2>&1 &

# Or if installed in /opt/cellular
sudo nohup /opt/cellular/auto-recover.sh 30 > /var/log/cellular/auto-recover.log 2>&1 &

# Set custom log directory via environment variable
export CELLULAR_LOG_DIR=/var/log/cellular
sudo -E nohup ./auto-recover.sh 30 &

Monitor the daemon:

# Default location
tail -f ./auto-recover.log

# Or system log location
tail -f /var/log/cellular/auto-recover.log

Automating at Boot Time

Option 1: systemd Services (Recommended)

Create /etc/systemd/system/cellular-connect.service:

[Unit]
Description=Cellular Modem Connection
After=network.target
Wants=network-online.target

[Service]
Type=oneshot
ExecStart=/opt/cellular/connect-cellular-robust.sh
RemainAfterExit=yes
StandardOutput=journal
StandardError=journal

[Install]
WantedBy=multi-user.target

Note: Update ExecStart path to match your installation directory.

Create /etc/systemd/system/cellular-auto-recover.service:

[Unit]
Description=Cellular Auto-Recovery Daemon
After=cellular-connect.service
Wants=cellular-connect.service

[Service]
Type=simple
ExecStart=/bin/bash -c 'exec /opt/cellular/auto-recover.sh 30'
Environment="CELLULAR_LOG_DIR=/var/log/cellular"
Restart=always
RestartSec=10
StandardOutput=journal
StandardError=journal

[Install]
WantedBy=multi-user.target

Note: Update ExecStart path and CELLULAR_LOG_DIR to match your setup.

Enable and start both:

sudo systemctl daemon-reload
sudo systemctl enable cellular-connect.service cellular-auto-recover.service
sudo systemctl start cellular-connect.service cellular-auto-recover.service

Check status:

sudo systemctl status cellular-connect.service
sudo systemctl status cellular-auto-recover.service
sudo journalctl -u cellular-connect.service -f
sudo journalctl -u cellular-auto-recover.service -f

Option 2: rc.local (Alternative)

Add to /etc/rc.local before exit 0:

/opt/cellular/connect-cellular-robust.sh >> /var/log/cellular-connect.log 2>&1 &
sleep 5
export CELLULAR_LOG_DIR=/var/log/cellular
nohup /opt/cellular/auto-recover.sh 30 >> /var/log/cellular-auto-recover.log 2>&1 &

Note: Update paths to match your installation directory.

Troubleshooting

Script fails with "Failed to retrieve IP configuration"

Cause: The modem hasn't finished connecting yet

Solution: Increase the sleep time after mmcli -b $BEARER_ID --connect:

sleep 5  # Increase from 2 to 5 seconds

"Cannot reach 8.8.8.8" but other connectivity works

Cause: Carrier may block external traffic or have firewall rules

Solution: This is normal for some carriers. Test with your actual server instead:

ping -I wwan0 your-server-ip

Interface shows "DOWN" after script completes

Cause: Interface needs to be brought up after configuration

Solution: The script already does this, but you can manually verify:

sudo ip link set wwan0 up
ip addr show wwan0

DNS not resolving

Cause: DNS server may be unreachable or incorrect

Solution: Check /etc/resolv.conf and verify DNS:

cat /etc/resolv.conf
nslookup google.com

Auto-recovery daemon not working

Cause: Daemon may have crashed or not started

Solution: Check daemon status and logs:

ps aux | grep auto-recover.sh
tail -f /var/log/cellular/auto-recover.log

Restart the daemon:

sudo pkill -f auto-recover.sh
export CELLULAR_LOG_DIR=/var/log/cellular
sudo -E nohup /opt/cellular/auto-recover.sh 30 &

Connection drops periodically

Cause: Modem may need reset or bearer may be unstable

Solution: Use emergency modem reset:

sudo bash /opt/cellular/reset-modem.sh
sudo /opt/cellular/connect-cellular-robust.sh

Note: Update paths to match your installation directory.

Then start auto-recovery daemon to prevent future drops

Monitoring the Connection

Check modem status

mmcli -m 0

Check bearer status

mmcli -b 1

Check interface configuration

ip addr show wwan0
ip route show dev wwan0

Monitor in real-time

mmcli -m 0 -w

Test connectivity

ping -I wwan0 8.8.8.8
curl --interface wwan0 https://httpbin.org/ip

Script Comparison

Feature Dynamic Robust Auto-Recover
IP Configuration Manual extraction Delegated to helper N/A
Modem State Management Basic Advanced N/A
Bearer Cleanup No Yes N/A
DNS/Routes Manual Delegated Monitors
Continuous Monitoring No No Yes
Connection Recovery Manual Manual Automatic
Best For Troubleshooting Production High Availability
Reboot Reliability ✅ Works ✅ Works ✅ Works
Error Handling Comprehensive Comprehensive Comprehensive
Logging Detailed Detailed Continuous

Technical Details

IP Address Format

The carrier assigns addresses using CIDR notation:

  • Address: 10.19.145.184 (your device's IP)
  • Prefix: 28 (network mask, /28 = 255.255.255.240)
  • Gateway: 10.19.145.185 (carrier's gateway for this connection)

The /28 prefix means:

  • Network: 10.19.145.176/28
  • Usable IPs: 10.19.145.177 - 10.19.145.190
  • Broadcast: 10.19.145.191

MTU Configuration

The MTU (Maximum Transmission Unit) of 1430 is typical for cellular:

  • Standard Ethernet: 1500 bytes
  • Cellular (with overhead): 1430 bytes
  • Setting correct MTU prevents packet fragmentation

DNS Configuration

The carrier provides DNS servers (172.26.38.2):

  • These are the carrier's DNS resolvers
  • May be different from public DNS (8.8.8.8)
  • Script uses carrier's DNS for reliability

Recommended Setup Path

For Production Deployments

  1. Test the robust script:

    sudo /opt/cellular/connect-cellular-robust.sh

  2. Verify connectivity:

    ip addr show wwan0
ping -I wwan0 8.8.8.8

  3. Start auto-recovery daemon:

    export CELLULAR_LOG_DIR=/var/log/cellular
sudo -E nohup /opt/cellular/auto-recover.sh 30 &

  4. Reboot and verify it still works:

    sudo reboot
# After reboot, check:
ip addr show wwan0
mmcli -m 0
tail -f /var/log/cellular/auto-recover.log

  5. Set up automatic startup using systemd services (recommended)

  6. Monitor logs to ensure it's working:

    sudo journalctl -u cellular-connect.service -f
sudo journalctl -u cellular-auto-recover.service -f

For Troubleshooting

  1. Test the dynamic script for detailed inspection:

    sudo /opt/cellular/connect-cellular-dynamic.sh

  2. Check modem status:

    mmcli -m 0
mmcli -b 1

  3. Review logs:

    sudo journalctl -xe

  4. Use debug script for comprehensive diagnostics:

    sudo /opt/cellular/cellular-debug.sh status
sudo /opt/cellular/cellular-debug.sh test

Support

If you encounter issues:

  1. Check modem detection:

    mmcli -L

  2. Verify ModemManager is running:

    sudo systemctl status ModemManager

  3. Check system logs:

    sudo journalctl -xe

  4. Test with minicom (for AT commands):

    sudo minicom -D /dev/ttyUSB2
AT+CPIN?
AT+COPS?