I have a Fibocom L850-GL LTE modem on my ThinkPad X1 Carbon 6th gen running Arch Linux. The modem is recognized, but the SIM is not detected. Here's what I've tried and the relevant information:

Hardware and context :

• LTE modem: Fibocom L850-GL.
• Machine: ThinkPad X1 Carbon 6th gen under Arch Linux.
• Main error: The modem does not detect the SIM card, even though it is correctly inserted. ModemManager returns error messages indicating that the SIM is “missing” or that the AT command failed due to a timeout.
• System logs :
  • Recurring errors concerning the wwan0at1 and wwan0at0 port, which “timeout” when an AT command is sent, such as the ATI3 command.
  • The modem goes from “unknown” to “locked” to “failed” due to the detected absence of the SIM.

Actions have already been completed:

• Check available tty ports with dmesg and the lsof command to see which processes are using the serial ports.
• Attempts to send AT commands via mmcli that fail with a timeout.
• Debugging enabled in ModemManager, showing serial port in use but commands failing.

Points to clarify:

• The modem seems to be recognized, but there are problems communicating with the SIM card, either due to a hardware problem (SIM port connection, modem) or a software problem (drivers, serial port management, permissions).

Summary

• The modem appears in ModemManager, but it transitions from "unknown" to "locked" to "failed" due to a missing SIM.
• I'm certain the SIM is inserted correctly and functional. (I tried it on another Thinkpad x1 7th, under Windows 11. It works perfectly.)
• Running mmcli -m 0 --command="ATI3" results in a timeout error (GDBus.Error.Serial.ResponseTimeout).
• Debugging ModemManager shows that the modem tries to open the serial port /dev/wwan0at0 but fails to run the AT command due to a serial command timeout.
• dmesg shows that the iosm driver is loaded and the device is recognized.
• I've tried resetting the modem using mmcli -m 0 --reset, but it reports that the operation is unsupported.
• Permissions seem correct, and I'm part of the dialout group.

Relevant logs:

ModemManager[14586]: [modem0] processing user request to run AT command 'ATI3'... ModemManager[14586]: [wwan0at0/at] opening serial port... ModemManager[14586]: [wwan0at0/at] device open count is 1 (open) ModemManager[14586]: [wwan0at0/at] --> 'ATI3<CR><LF>' ModemManager[14586]: [modem0] failed running AT command 'ATI3': Serial command timed out ModemManager[14586]: [wwan0at0/at] device open count is 0 (close)

What could be causing this serial command timeout, and why is the SIM not being detected? Is there an issue with the iosm driver, or could it be a configuration problem with ModemManager or the serial interface?

The modem was stuck with repeated errors such as SIM not inserted and timeouts on serial ports with ModemManager.

Solution:

1. I used the script open_xdatachannel.py from the xmm7360-pci library to open the serial port that was blocked xmm7360-pci GitHub Repository once this script was executed, the serial port opened correctly.

2. After that, I restarted ModemManager with the command :

   sudo systemctl restart ModemManager

3. The modem then correctly detected the SIM card and connected to the network.

Conclusion

The problem was linked to the opening of the modem's serial port, which prevented ModemManager from operating correctly. Running this script solved the problem and established the connection.