Happy Sunday! Bare with me - I’m not overly savvy here…I have a Hisense TV that came with Plex preloaded. I can log in, see the library, select a show, watch the trailer but i cannot watch the full length show. I just get a black screen with rewind, fast forward etc at the bottom. I’ve tried changing to H.264 but still cannot play. I’ve installed the app to my ipad and iphone and can watch fine on those devices but I can’t even cast it from my phone - the hisense selection isn’t there. But i can cast YouTube. Any help would be appreciated. 
Sorry to say, but Plex does not officially support the hisense tv’s. Plex was not involved in making the app available on these TVs so we have not tested our app for compatibility.
As a wild guess, it could be an incompatibility with the security protocols we use. You can try turning security off in pms and see if that helps.
