I am new to Plex and just installed Plex. I cannot find out why my new LG TV is greyed out under 'Authorised Devices'. Any help appreciated

Dude you joined at the worst time lol. LG TV’s and Web OS are currently broken. See this thread LG WebOS 6 Server Unavailable - #13 by RustyNail

I would recommend using an external streaming device and let the TV be a TV if you get what i mean.