You need to connect to your alternative Roon Server first - then you should get the screen that asks you to Deauthorize the Roon Server installed on your QNAP.
Is this not happening for you? Then some screenshots of what you are seeing would be helpful for further diagnosis.
Yes, nothing happens, the Roon Server is running on my QNAP TS653. The new one on a NUC in my local network
It seems there’s no connectivity to internet. but nothing should be blocked.
Yes, that message states that Roon is having problems accessing the internet… Are you sure that all the necessary Firewall rules are in place on your devices?