Unfortunately, this TV won’t be usable to stream from remote Plex servers anymore. It only “speaks” an old, broken, and insecure communications protocol, which recently has been disabled at plex.tv
The only way to restore access to remote Plex servers would be to either:
replace the TV with a newer model
add a streaming device with Plex client (e.g. Apple TV, nVidia Shield, Roku, FireTV …)
