DSM version 6 or 7?
Verify your PMS version - 1.41.x, 1.42.x, etc. Look in Settings → Server_Name → General.
Can you access the Plex server OK when on your home network?
Remote access is never working. Plex “optimistically” says it is available while trying to register your server with plex.tv. When that fails it returns to “not available…”
This document can help with getting remote access working: The Basics of Remote Access Troubleshooting
