Unless you have an older or non-4K model, you received bad information.
Plex supports LG 4K/UHD TVs running webOS 3.0 or higher.
The LG Photo & Video app uses DLNA to stream from Plex Media Server.
Monitor the RAM usage on your server. A couple of years ago, there was a bug in Plex Media Server where using DLNA with LG TVs would slowly (over days) consume all available RAM on the Plex Media Server. I do not know if the bug was ever resolved.