I think you need to update the APN info in settings/mobile networks/access point names... (at least that is where the settings are in cm-10.2. I can't remember where they are on the stock ICS rom)
Google for AT&T APN and you should find something... you can probably also look on XDA in some of the other AT&T device forums.
Google for AT&T APN and you should find something... you can probably also look on XDA in some of the other AT&T device forums.