Have you checked your APN settings? I would also try power cycling and if all else fails try contacting ATT. If you dont have any apns, search around and you should find them.
Moto Backflip(stock)----Samsung Captivate(Rooted, Custom Rom)-----Moto Atrix(Rooted, Stock Rom)-----Samsung GS2 Skyrocket(Rooted, Custom Rom)------Samsung Note 2!!!!(Rooted, currently running CM10.1)-----Samsung Note 3(Stock, for now!)
Viewsonic G tab(Rooted and so, so many Roms)----N7!!!(Rooted)------N7 V2(Rooted)