As I've advised other who bought AT&T phones:
I cannot emphasis this enough. (assuming it was labeled as AT&T locked, if it wasn't, you may want to contact the seller)
DO NOT PURCHASE SECONDHAND AT&T-LOCKED DEVICES WITH THE INTENT TO UNLOCK OR USE OUTSIDE THE US.
AT&T as a policy does NOT unlock secondhand/used devices. Even if you were the original owner, they will require 60-days of their service on their device I believe.
If you were to steam ahead with that specific device, unlocking via a third-party channel ranges from expensive to impossible.
The fact you're outside the USA doesn't help your problem either.
I will say the best course of action (from my point of view) is to sell it to someone who can put it to use in the US, and then obtain a different unlocked device or device locked to your carrier of choice.
Good luck, you'll need it.