modem_cellular: a lost network registration causes network to hang

[emillindq]

Describe the bug Using modem_cellular with U-Blox SARA-R5 and U-blox LARA-R6. All works well until the modem reports lost cellular network registration. This causes a transition in states from CARRIER_ON, via a call to net_if_carrier_off(), to enter state RUN_DIAL_SCRIPT causing it to send AT commands again on the PPP DLCI channel.

Personally I don't see how this could work, because the DLCI channel assigned to PPP is still in the data call, only speaking PPP. The modem of course doesn't reply and so we're stuck in a timeout loop.

According to the AT command manual (p. 236) the modem exists the data session once

• Sequence "+++" is sent
• DTR pin transition
• Sending an LCP Terminate Request And we never send LCP Terminate request.

I'd really appreciate some input from @bjarki-andreasen here what the idea about what should happen is, because I don't understand this. Is it working on Quectel BG95? Thanks!

NOTE: I am using PAP

To Reproduce

1. Build application using modem_cellular.c driver
2. Gain internet access over 2G/3G/4G
3. Disconnect antenna, or mock a deregistration event

Expected behavior Once network registration comes back, network access should recover and a data call can be placed once again

Impact Unrecovered cellular connection; need to use external watchdog-ish to suspend & resume modem

Logs and console output State transitions (no output on the UART)

Environment (please complete the following information):

• e60da1bd640a37370870a83277142dd560f1fb8d

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