Are you using the Tailscale setting in Channels? I have found using Tailscale on my server works best. It seems Tailscale is a bit slow inside channels. Also, I use Tailscale via Starlink in my RV and there is no buffering. Also at my daughter's house via T-Mobile. She did have bandwidth issues until they improved the cell site nearest her. I run original quality via both. That is usually only 5 to 7 Mbps on all except local channels. I try to us TVE for everything to keep the data needs modest.
However, I use fiber at my house which is 500 up/down. I suspect your T-Mobile is choking your outbound from your server. I would try some different locations for you T-Mobile router to see if you get improved transfer rates.