Why does TMobile Home Internet throttle only Live TV apps on FireTV Cube?

Why does TMobile Home Internet throttle only Live TV apps on FireTV Cube? All other streaming services (Netflix, Hulu, Prime etc) work fine. My LiveTV iptv app buffers every 15 sec then freezes for 2 or 3 minutes, then buffers again, repeat this sequence over and over. This just started happening this weekend. I've had this same TMobile Home Internet & the apps mentioned for 2+ years. I talked to customer service who called in "expert engineer" to diagnose, said shed call me back - its been 2 days, no call back.  Can anyone help me?