I've a RUT 360 with latest firmware 7.1.4 installed; I used the router just with a LTE card to connect to the internet.
After entering the PIN it first seams that the model im connection, cause the router has been registered. ´But 15 to 20 seconds later the modem has hung up .. and the router says, that no sim card is inserted. Also the firmware of the model is not displayed and it seams that the router has not mobile modem anymore. This modem will not occur again (even after reboot), if the PIN is present. Just if an empty Sim Card PIN is entered and the Router is restarted. Troubleshoot-Zip created; can be send to you per PM.
Also tested to reset the router completely; but that also has no effect.
Even firmware downgrade to 7.0.2 and and upgrade 7.1.4 without keeping settings did not solve the problem.