There’s offical Plex addon for Matrix (Kodi19, which is the Kodi version used on CE19) maintained by people from Plexinc themselves, you should be able to install it from Kodi repository directly, there’s more info for the addon here: Plex | Matrix | Addons | Kodi
I didn’t use the addon though, you might need to ask in the plex for kodi catogery on the plex forum (link also in the above URL) for further help