I will be moving to the US shortly, and so I have been searching up about phone compatibility. Until I decide on a new phone, I will be using my Lumia 920 (unlocked) that I bought overseas. Below is the relevant information about my Lumia 920. Can I connect to AT&T's LTE/4G/3G?
 I also ran into an article on Reddit that shows a guy from Singapore with an International Lumia 920 and his experience on T-Mobile, and on AT&T.
And from his review, he states that he cannot use LTE. But that he is able to use 4G.
quote from article:
... AT&T however uses the 700mhz band for the majority of their LTE. The Lumia 920 is missing this band and as a result I have never been on their LTE network despite trying several different APN settings with the assistance of their tech support. AT&T does support the 2100mhz band which the Lumia 920 has but I suspect they have not done a huge deployment on this band yet as I have yet to connect. ...