I would call ATT and report your network issue and/or you possibly have a hardware issue. Do others near you with ATT have the same issues? If not have your phone checked out at an Apple store. www.Apple.com/retail or have an ATT corporate store swap your SIM for a new one it may have gone bad.
For service or support questions including existing order status, call: Customer Support: 1-800-331-0500
Information on iPhone unlocking can be found here by copying this link into your browser: https://www.att.com/deviceunlock/client/en_US/