you cannot access your Plex server from the Samsung app
Which version of the Plex app do you have installed?
Generally speaking, the Smart Hub platform (=old Samsung clients) is dead. You’ll need a TV with the Tizen platform (Tizen 2.4, 3.0, 4.0, 5.0 or 5.5) – usually this is Samsung 4K/UHD models from 2016 or newer.