Changing your Plex password unclaims yours server (removes it from your account).
You need to reclaim it.
Option 1: Point a private/incognito browser window at the IP address of the NAS. Using http, login and claim the server. For example, if the IP address is 10.1.2.3, use http://10.1.2.3:32400/web.
Option 2: FAQ 31: Reset / Reclaim PMS on DSM 7 (Not Authorized - Locked Out)
