You need to re-claim the server. After re-claiming, you may need to restart the client or logout/login to the client app.
From a system on the same network as the NAS, point a browser at the IP address. For example, if the IP address is 10.1.2.3, use http://10.1.2.3:32400/web (use http). Do not use QuickConnect, a domain name, etc.
Login, select the wrench icon (settings). The server should be listed on the left, under the Plex Web section. If so, select General (for server, not Plex Web). Claim the server.
If that does not work, see the Synology FAQ:
