Can't access newly installed Plex server on synology NAS

The server never claimed to your account.

There are no errors . It just didn’t claim.

Because you didn’t claim, you can’t use Quick Connect yet.

'192-168-0-154.brokenXXXXXXX.direct.quickconnect.to' in header Host; treating as non-local

When you claimed the server, did you get the return result with your username?

Lastly, do you have the Syno assigned to a FQDN ?