iPhone X bought at Best Buy - US

Hello, i´ve bought an iPhone X at Best Buy, i understand such device is subject to the US Reseller Flex Policy.

I have a T Mobile SIM, that seems accepted for activation, if it try to activate using the local Brazil carrier it fails to activate with a "SIM Not Valid" message.

From the US Reseller Flex Policy, as i activated it with a T Mobile SIM, i should contact T Mobile for unlocking, hovewer, i did contact them and all i got is that "the device should be already unlocked". Still, my local Brazil carrier SIM does not work.

Any clues?