1. Use AT+CPIN to check whether module can recognize the SIM card
2. Check whether the SIM card is out of service
3. Use AT+QBAND and AT+CFUN to check the working band and working mode of the module respectively
4. Use AT+CSQ to check whether the RF signal is normal
5. Use AT+GSN to check whether the module’s IMEI number is legal. Changing the IMEI number can cause networks in some areas to treat the changed IMEI number as illegal and prevent the module from registering with the network