When you uninstall, one thing which doesn’t happen is that Plex.tv is not notified.
That leaves the ID numbers of the server you just uninstalled as “active”.
When you now install again, Plex/web is looking for the old ID numbers at that IP address but it’s getting the ID numbers of the new server you just installed.
Now Plex.tv, Plex/Web, and PMS will be in sync again.
Given you don’t see the old server under Authorized Devices (which is unusual), just skip that step and make certain to log out of Plex/web and continue to open the QNAP via http://ip.addr.of.qnap:32400web
There was no server instance in the bar on the left (as you can see) so I skipped that part too.
I then signed out of plex and closed the tab.
After opening a new window with the plex web page it asked me to sign in again and I get the online services on the homepage but no mention of my server in the navigation bar on the left and if I go to settings no mention of any servers in the list on the left - any ideas?
Yes, I went via app.plex.tv to remove the authorised devices, signed out and then opened a new window with http://IP.addr.of.QNAP:32400/web.
It turns out your instructions were right and I was following your instructions correctly BUT The issue was with your comment there around the subnet - I was trying to access the page from a local but different routed subnet.
I’ve now connected to the same subnet as the server is on and its taken me straight to the setup page and I’m through into operation.
Sorry if I missed that part in some other instructions but why does it behave in this way? Is it security?