Unlocked iPhone 6 report lost/stolen

hello guys.

well I have a problem, I recently bought an iPhone 6+ from AT&T but it had already been unlocked by them it was usable in any carrier, but the guy who sold it to be reported it lost, and it's now in the blacklist, and I can't use it with my AT&T SIM card, my question is, is the phone worthless now? And if it's possible that it still works with another carrier i will be moving to Mexico and I wonder if I could use it over there, thanks.