Instead of chasing rare firmware for an obsolete module, consider:
: Facilitates VoLTE (Voice over LTE), MMS, and SMS functions essential for IoT terminal operations. Industrial Compatibility
The modem reports +CGEV: NW DETACH or +CEREG: 0 every 30–90 minutes. This is a known bug in early ML7820 firmware (versions below 05.05.05.00 ) related to RRC (Radio Resource Control) state machine errors.
Identifying and reporting hardware hangups or communication timeouts.