My ADH8066 responds normally only to command AT+CSQ (+CSQ: 8,99). All other commands return “ERROR”, even basic stuff like AT+SFUN=OFF. I do not have a SIM card in yet, but I thought system management commands (set baud rate, power off, get firmware version, etc) should still work. Not so?
Figured it out. It’s true, without a SIM card the module won’t even obey the power-off command (AT+SFUN=OFF). As it turns out, there is one other command it would obey without a SIM inserted, and that is the Modem Error Reporting command (AT+CMEE=2). 2 = verbose errors; handy for humans.