ATT GS4 Moved to T-Mobile and hotspot topic

So my friend recently just moved over to Tmobile from ATT. He unlocked his ATT GS4 and with Tmobile the hotspot is enabled on the plans however in this case the app gives an error when he tries to turn it on.

So, how can we get the hotspot functioning on his ATT GS4 thats now on Tmobile?


TIA