Unlocked AT&T iphone 4 iOS 7 issues

I'm in need off some major help for some reason my apn setting are not saving to my iphone I'm currently on iOS 7 AT&T iphone 4 unlocked and using tmobile sim. Well it was working find for about two weeks and now nothing no service at all I spoke with apple and tmobile on the phone together and they could not figure out the problem they told me what apn settings to input and I did everything they told me to do but apn settings are not saving any help would be nice and thanks in advance