How To Fix a Quectel EM120R-GL LTE Modem On Ubuntu

This guide documents a working fix for a Quectel EM120R-GL PCIe LTE modem on a Debian/Ubuntu style Linux system using NetworkManager and ModemManager. It was validated on a machine with: Quectel EM120R-GL Device ID 1eac:1001 A Sri Lanka SLTMobitel SIM NetworkManager ModemManager The same pattern may also apply to other Quectel MBIM modems that are fcc-locked on Linux. Symptoms The modem is physically present, but mobile data does not come up on Linux even though the same SIM and modem work in Windows. Typical signs: lspci -nn shows the modem, for example: 05:00.0 Unassigned class [ ff00]: Quectel Wireless Solutions Co., Ltd. EM120R-GL LTE Modem [ 1eac:1001] mmcli -L shows no modem because ModemManager is not running, or it shows the modem but activation fails. nmcli device status shows the WWAN device as disconnected or stuck connecting. The ModemManager journal shows repeated messages like: couldn't enable interface: 'Invalid transition' The raw Quectel MBIM vendor state shows: sudo m