youtube tv can't determine my location with my t-mobile gateway

To understand what you can do, you first need to understand why.

As noted this is mostly due to wireless carrier’s infracture. Unlike wire line carriers (e.g. cable) they really don’t have the ability to assign IP addresses by location - there simply aren’t enough due to the mobile nature of the services. At the same time, unless the device you’re streaming to has GPS (TV’s don’t yet) the content providers will rely of IP-based geolocation services which were always a hack anyway. IP addresses don’t say anything about your location other than country. Due to the nature of wire line services (before mobile became prominent), these the “wizards” at these  3rd party geolocation figured out a way to provided somewhat accurate locations for those service wire line services based on some wild guesses and some are wilder than other when it comes to mobile and satellite services.

Some content providers now rely upon browser based JavaScript apis (usually in conjunction with the specific browsers services). FWIW, YouTube TV does utilize that JavaScript api to determine location and it tends to be accurate; however, in my case I utilize Firefox and it’s not so accurate unless using the Location Guard add-on extension which allows me to establish my proper home location for YouTube TV. I then stream to my “dumb” TVs or monitors using an HDMI cable from the PC to the TV/Monitor.

That’s one way; however I also use a second approach that actually works out better using the YouTube TV app on my phone and a USB-C to HDMI cable (actually a powered hub) from my phone to the TV. When used this way the app is picking up the phone’s GPS location - this does require a phone  having HDMI capabilities. I personally utilize a high-end Samsung flagship coupled with Samsung’s Dex (desktop experience). Works so well that I doubt I’ll ever buy another PC or workstation again - gives me a desktop experience when coupled with a bluetooth mouse and keyboard; and can easily take it on the road and there’s always a TV around with an HDMI port free. The YouTube TV app on my phone never fails to locate the proper local channels regardless of where I am. The same also holds for Hulu Live and other apps I’ve tried (I simply prefer YTTV)

Sorry for the long response but to tackle it you need to know what’s going on - it’s really not T-Mobile’s problem but that of content providers who rely on IP based geolocation services which are notoriously inaccurate. Understand the problem to solve it and much more.

P.S. I suspect YTTV is making some changes to how location is determined now that NFL Sunday Ticket has arrived. Careful what you wish for. Undoubtedly, those changes will impact all of YTTV - gotta protect those out of market profits for those bilionaire NFL owners ya know!