Unlocked iPhone 6s but Sim not supported by carrier issue.

Hello,

I have a AT&T iPhone 6s that I unlocked online and was approved. I followed the instructions on the unlocked form and updated it through iTunes  and apple software and all the other approaches but I'm still unstable to use my T-Mobile SIM card. Contract is all paid out and nothing seems to be pending.

It says the Sim card is not supported by the carrier. I have the approval form if that helps but is there someone that can do another check to see if it's truly unlocked. The email says it is but the iphone says otherwise. Or if anyone knows what the issue could be, any feedback or suggestions would be great.

Thanks!