Add a cron entry on your Ubuntu server.
0 4 * * * rsync -r user@seedbox:folder/ /my/plexfolder
This would sync the folder on seedbox to the /my/plexfolder every night at 4am.
Ensure that you have set up password less authentication with ssh keys.
It you have a server, you could ditch streamio and run Plex with debrid. That way you could all stream remotely as well.