I’ve had intermittent issues with T-Mobile on hotspot too. I’m not sure how helpful this will be but here’s my 2 cents. The only params I played with that seemed to help were the : 1- MTU (if I remember correctly, I had to dial it down to 1300) 2- and using IPv6 instead of v4. This will depend on the APN you’re using for T-Mobile. I believe they have a legacy one that only uses IPv4 whereas their new one supports IPv6 only (I wasn’t able to find clear info about this but this is my guess). In any case, I have my wireguard server setup to support and use both IP versions and when v4 doesn’t work for me, switching to v6 fixes the issue for a while. At some point I was starting to suspect that they were simply heavily throttling wireguard traffic which may be the case but who knows.

I hope this helps, good luck!