It creates a legacy Wi-Fi direct group and runs a proxy server on the phone. You can then connect via the proxy in some programs on your computer (e.g.: Firefox).
You can also run a proxy server in Termux. I use tinyproxy because it's easy to setup.
Both of these will also go over VPN if you have that connected on your phone, as it's basically the app (proxy server) using internet on behalf of your computer.
Alternatively you can also use socat for port forwarding, for example to a public VPN server. Then on the computer you could set up the VPN connection as usual, but replace the IP/Port with those used by your phone locally.
Careful doing just this. They actually track http request headers and look for desktop signifiers, so your best bet is to use a VPN in tandem with whatever setup you go with. (Tried doing this a few years back without the vpn, they sent me some very upset texts.)
I use desktop mode all the time on my phone's browser because mobile sites are too limited. How can they just assume nobody is doing this? Sometimes I even use desktop version of Firefox in Termux because mobile browsers tend to crash large uploads with locked screen.
But I guess I shouldn't be surprised if they already do this hotspot usage limiting. Greed has no limits.
Mate I’m genuinely not trying to be an ass with this but does your plan specifically include mobile hotspot usage?
T-Mobile has dozens and dozens of different plans from the last decade alone, it’s quite possible your plan has zero hotspot data. Especially as you mention it’s an older grandfathered plan.
Have you tried some basic network tests from the device that is connecting to the hotspot?
Since a hotspot is turning your device into a router, simple things like ping or traceroute can start to tell you some basics of if the configuration is correct.
Also check your connected devices default gateway and make sure it matches your wifi hotspots IP as given to your connecting device.
EDIT: I just checked and there are indeed SOME plans from T-Mobile that don't include support for hotspots. Check your plan to make sure, and then try the workarounds from /u/lukmly013