Only Hisense running the U4 system software is supported. Plex for VIDAA (Hisense televisions) - #30 by markus101
Your screen shot indicates that the Plex app on your device is ancient and is not supported anymore.
The last nail in its coffin was the recent expiration of an important security root certificate: Important information about Plex for smart TVs after September 30, 2021
