Plex (player?) missing from list of apps in Asustor Portal

Yes, this is pretty much the default mode of operation with Plex.
Server and client on separate devices.

Sorry, I don’t think there will ever be a client app for a Nas device.

4k is still a challenge.
Give this a thorough read: [INFO] Plex, 4k, transcoding, and you - aka the rules of 4k