How was your iPhone unlocked originally? When your iPhone
locked after restore, that indicates it was jailbroken or hacked
to work because if AT&T unlocked the iPhone it would still
be unlocked. Contact whoever unlocked it originally and ask
for their assistance. Or contact AT&T who now offers unlocking
if you meet their requirements.
It is not the wireless carrier's responsibility to warn you that the
update might lock your iPhone. You assumed responsibility when
the jailbreak was applied. Updates are written for and tested to work
with the normal software that is found on the iPhone, not for
software that was altered to perform functions not intended by
original OS and hardware combination. Carrier had no
knowledge the iPhone had been altered and therefore could not
warn you against possible locking.
Google may assist you in finding a solution, but these fora cannot.