The 535 and 535 Dual SIM are not really constructed for the US market, radio-wise. EDGE only would be your usage either on T-Mo or AT&T towers... No matter if you use them direct or through an mvno (airtime reseller). They both lack the proper 3G radio needed.
If you want AT&T service on a budget phone more modern than 520... Look into Blu Win Jr.
BLU Win JR - Full phone specifications
It's priced attractively and will get you Flash on the rear cam and a front cam to boot. You need the w410u model.
Buy BLU Win Jr - Microsoft Store