Are you about the “Live TV Streaming on Plex” or streaming OTA live TV from a connected tuner (Plex Live TV / DVR)?
The (online) Live TV channels appear not yet to be available on LG smart TVs
https://support.plex.tv/articles/free-live-tv-streaming-overview/
If you have setup an own TV tuner for live TV / DVR in your Plex Media Server, that should be working with your LG TV. However… when the LG preview started the new Plex app apparently didn’t have support for DVR scheduling (I cannot currently check this or find it mentioned in the subsequent preview release announcements)
https://support.plex.tv/articles/115007689648-watching-live-tv/
