I think I finally found an answer on how to properly reset the modem in case it just won't attach to a network. Reset the modem by sending following AT-command: lte.send_at_cmd('AT+SQNSFACTORYRESET=?') Unlike lte.factory_reset() - which seems to use AT-command AT+SQNSFACTORYRESET - this command will successfully reset the modem to factory state. A reboot of the device is necessary to finalize the reset! After this process the modem should attach to the network without any issues! Also, for further information see: AT Commands Reference Manual, 15.1 Device Reset to Factory State