Sim card isn't supported

If the unlocking portal doesn’t work, there isn’t much that can be done.  You can try calling them, but if you’re not an AT&T customer and bought a phone from a third party, they probably won’t be very helpful.  Just tell you to keep trying the portal.  You can try filing a BBB or FCC complaint, but those may not work It you’re not in the U.S.

You can try using a US VPN to access the unlock portal.   As a non USA resident, and non AT&T customer, you don't have any other option other than the portal.  

Make sure you fill it in carefully, filling in the "no" bubble..   If the unlock is denied, you are stuck

Where did you get the phone? If the seller was an AT&T customer ask them to unlock it for you.

AT&T is extremely unlikely to help you, you can't prove the phone is eligible to be unlocked as a non customer