Hmmm… from what I could find, the recent change by Plex to no longer support outdated connections should only impact Samsung TV models with Plex version 1.x. Then again… no longer being able to sign into your account from the TV sounds a lot like that.
The SmartHub app had been maintained by a user who stopped developing it. Plex’ own Samsung app is only supported on from 2016 or newer models.
Your best shot might be to add some kind of streaming box to the TV – you could check out an entry-level Roku or Chromecast stick to keep the TV “alive” even if the native Plex app is giving you trouble.
