Plausible cause.

-Carrier Locked

-Damaged Sim Reader, really difficult especially with Phones.

-Carrier Settings need to be updated.

*Leave sim in unit, connect to wifi, go to Settings > General> About. This should trigger a carrier update, or simply updating the phone.

-Check Settings > General > Scroll down to Profiles

*Should be in the same area as VPN, and see if a profile from the previous user is interrupting the service (This is primarily dependent on your region/carrier service).