A huge draw of Plex is the ability to stream the media remotely, and for the ability to have the server automatically convert the video into a format playable by your local device. If both of these are not necessary, then an alternative (Kodi) might be advisable. Kodi is a “play-it-all” video player that has no issue playing most videos (assuming your device is capable of decoding it fast enough). It can play videos over the local network as well, which sounds like a good fit.
That being said, I suspect you are running into an older-TV-certificate error. Plex relies on installed certificates in smart TV’s. Certificates expire (for some dumb reason). TV manufacturers need to push a software update to TV’s to update certificates. They stop doing this for older devices. See this article for info.
Edit: That-being-said that being said, I forgot to mention that it is possible to disable most authentication requirements for the server, following this guide.
