Can't add my server to existing Plex account

Plex Media Server NAS & Devices
1

I can’t add my plex server to my existing plex account. I’ve uninstalled and reinstalled the plex app on my NAS multiple times and nothing changes. It used to work perfectly, but after after spinning up my NAS after moving I can’t access or even view my server from plex. Whenever I log into plex I get the “Looking for Servers” message, but I can’t manually add the IP address.

2

what do you mean you cannot “manually add the IP address”?
can you access the server via Plex Web (https://your-qnap-ip-address:32400/web)?

have you checked out this instruction?
https://support.plex.tv/articles/200878643-sign-in-to-your-plex-account/