I have listed Some AT&T APNs and are different
US – AT&T – for older plans
US – AT&T (wap.cingular) – non smartphones usually
US – AT&T (isp.cingular) – smartphones usually only
US – AT&T (Broadband) – most newer smartphone accounts
US – AT&T (PAYG) – PAYG AT&T Accounts
So saying all this.. if it doesn’t work initially.. then try one of the others..