Unlocking an iPhone 6s that still has payments left from previous owner

Hello, I bought my iPhone 6s at swappa.com where it was att locked. My phone plan is under cricket so it was able to work even though it is att locked. I tried to unlock on the att unlock website but it said that I wasn't able to unlock it because I hadn't paid off all my payments for the phone. But, Im not able to pay for the payments because I didn't buy it from att. I bought it from the previous owner who I think bought it from att. So is there anyway that I can get it unlocked? The reason why I'm asking is because I'm going abroad in a few weeks and want to use another SIM card. Thanks.