My service can't connect

Hi everyone so when I first got my phone my service wasn't connecting and it was unlocked for my provider and I have only dropped the phone once but it was like only a foot up. And I have gone to AT&T but they can't help me. The phone is a hand me down and it worked fine for 2 years on Verizon wireless but then when I switched it to AT&T it started to work on and off and I've tried a lot of " Solutions" like taking out SIM card, network reset, restore to old backup, etc. so if anyone can help that would be amazing -Jack