If your BenQ M23 GSM module is losing network or doesn’t stay in the network you might have to register it to network with command:

AT+COPS=1,2,”24405″

Where 24405 is the “numeric format is the GSM Location Area Identification number which consists of a three BCD digit country code coded as in ITU-T E.212 Annex A[10], plus a two BCD digit network code, which is administration specific”.

You can get LAC number by commanding:

AT+COPS=?

(and wait for 10 seconds or so..)