Galaxy Note 2 - Forgot to change apn settings; can only make emergency calls. What do I do now?

[macdp59]

I purchased my Galaxy Note II on Amazon.com and bought the AT&T comp sim card.

Cut the card down to size and inserted okay.
Activated with StraightTalk. Worked fine for three days, now I can only make emergency calls.

I realize now that I should have changed the apn right away. What do I do now?