you’ll have to mount it and make sure the local user plex on your Linux setup has access to that mount. there’s a number of recommendations in the tips & tricks section on how to achieve this and what caveats to avoid (e.g. not to mount the drive in /media).
as for the connectivity… if it only “seems” correctly configured… maybe re-checking the firewall settings is worth a shot 
are both apps linked to your account as well?
