Sorry for the late reply; with COVID-19 raging here, I haven't been able to be around TMC much lately.
The "Denied Registration" message means the cellular network denied registration to the SIM card. This normally happens when the SIM tries to connect to the network before the SIM has been provisioned. The problem is that if this happens too many times (ie; SIM repeatedly tries to connect and cellular network repeatedly denies registration), according to the GSM standard the modem must block the SIM registration for some time to stop repeated registration attempts. In my experience, the block normally clears after 24 hours or so.
There is a low level AT commands to view the block, and clear it, in the SIMCOM modem.
The "Denied Registration" message means the cellular network denied registration to the SIM card. This normally happens when the SIM tries to connect to the network before the SIM has been provisioned. The problem is that if this happens too many times (ie; SIM repeatedly tries to connect and cellular network repeatedly denies registration), according to the GSM standard the modem must block the SIM registration for some time to stop repeated registration attempts. In my experience, the block normally clears after 24 hours or so.
There is a low level AT commands to view the block, and clear it, in the SIMCOM modem.
There must be exactly 24 "F" in that. After issuing the AT command, power off the modem, and power back on. Leave the device powered on for at least 7 minutes.AT+CRSM=214,28539,0,0,12,"FFFFFFFFFFFFFFFFFFFFFFFF"