Thanks for the warning! On my last trip a T-Mobile US prepaid SIM (great deal) worked just fine (3G only), but it seems AT&T is the last remaining option now.
What they are actually doing is switching off HSPA (3G) and deploying LTE (4G) on the same frequencies (band 2, 1900 MHz). While the FP2 does support HSPA on band 2, LTE does not work on this band. So it can now use neither 3G (defunct) nor 4G (band not supported) and falls back to 2G.
(They also use band 4 / 66 (1700 MHz) and 12 (700 MHz), but those are not supported by the FP2 at all.)
AT&T still operates an HSPA network on band 2.