This is likely to be a hopeless fight.  AT&T has historically fired customers who use their inclusive minutes or data in way they deem to be excessively off network.  They would rather close the whole account than continue to pay another network for what becomes ordinary and not occasional use.  In particular phones that rarely register on the at&t network get targeted.  A phone which has been operating full time in Mexico for 8 months seems rather suspect for targeting.

When I signed up for my international data plan two years ago I was verbally informed that it was for occasional and not full time use outside of the US.