If you do change the port in Plex, be sure to update the proxy as well at
You should be able to access it without going through your QuickBox dashboard. Additionally ensuring your tunnel is created and I usually just access it through app.plex.tv.
About the port on the dashboard, this is staged in your config.php file located at
/srv/rutorrent/home/inc. At line #295 you will see:
$plexURL = "http://" . $_SERVER['HTTP_HOST'] . ":31400/web/";
You can modify this forward here.
As per physically changing the default port (32400) of plex itself. I am not too certain. @liara may no more on that that I would.
According to some random entry on Plex support I found this:
The Local TCP Port number cannot be changed - it has to be 32400.
Only the remote port can be changed
However, seeing as you are wanting to only change the remote port... just be sure you're doing it this way: