TMobile use unbranded (GENERIC) firmwares. And the customization is made by a TMobile branded custom pack. So, this combination of firmware and custom pack is good.
First reflash your phone complelely. Here is the guide: XS++ 2.x Guide
i've pressed both flash as well as the unlock button
Where do you read that you need to press the Unlock/Repair button? I suggest you to follow carefully the guide on first post of this thread.