Get a Wifi signal, but no Cellular Signal

I got an iPhone 4S 32GB off of eBay with a cracked back and advertised as working perfectly. Well it seems that returning it is now impossible, as the seller shipped it with another issue that it wont connect to AT&T or T-Mobile (Its unlocked). Since I fixed the phone after this issue was found I can't return it, since I "tampered" with the device by replacing the cracked back glass with a new one (not really tampering if you ask me, but eBay must know everything! being sarcastic) but back to the point as far as I know the blue coil is there (could it just be bad?). Any help is appreciated. (sorry if this is a repost, couldn't find anything like this but close)

EDIT: Sorry forgot to add but bluetooth and wifi work, just no carrier reception.