Hey there! While it is possible for the hotspot to function as an internet source for your Xbox, it is not something that we can fully support. Hotspot service is meant to provide internet access to devices for more common data activities like browsing and streaming. When it comes to gaming, there are some additional network settings that may need to be configured for the games you play like NAT settings and these types of settings are not configurable when using a hotspot as your source of internet.
Agreed with above- T-Mobile can only troubleshoot issues with phones, tablets, and computers/laptop; I would try to connect with one of these devices to determine if the cause of the issue is your hotspot/network, or if it is because Xbox requires extra unprogrammable settings to be correct in order to function on the network properly; in which case it can't be helped because you can't change those settings using that type of connection. Your phone is not a typical "router" in that you don't have the ability to adjust things that you would a standard router that you'd use with a home internet connection. TVs, Roku, Firestick, Video Game consoles, etc are not supported by T-Mobile.