From what I am finding, there are three versions of the 925 (this is from gsmarena). It claims three models, the RM-892, the RM-893, and the RM-910. From what I can see, the 892 is the European version and does not support 3G (except in refarmed areas) or LTE on T-Mobile. The 910 is an HSPA+ only version of the phone.
The 893 appears to be the T-Mobile version of the phone and appears to support 3G on both AT&T and T-Mobile (has HSDPA 850 / 900 / 1700 / 1900 / 2100 frequencies; AT&T uses 850 and 1900, T-Mobile uses 1700 and 2100). As for LTE, they show the phone supports LTE 700 / 850 / 1700 / 1900 / 2100; AT&T uses 700 and 1700 (channels 17 and 4) and T-Mobile 1700 (channel 4).
So is gsmarena wrong, or am I missing something? Because if I am looking at the specs correctly, the T-Mobile 925 should work on AT&T (both 3G and LTE).