When watching live TV remotely (ie NOT using WiFi) Plex fails to change the quality automatically and stays on the “Remote streaming quality” you initially set. What it should do is adjust quality up or down according to bandwidth available.
This does NOT happen with anything recorded which automatically adjusts quality OK.
Android App settings:
Automatically adjust quality: on
Limit cellular data: off
Remote streaming quality: anything
Easy to re-create as per above, set any quality, begin playback then check during playback and the quality never changes.