If you send at+cmee=1 to the unit prior to sending the other commands then it will give you a cause value back, if you get 655 then the credentials on the unit are broken/not present and the AV server will not be able to authenticate with it. There is no way to load new ones on (even with SWI it is next to impossible) this can only be done at the factory so the best way is to get a new unit.