macOS cannot add a bare IP address or a server root to Login Items; it requires a specific shared folder (volume) to mount. This is expected behavior on macOS Golden Gate. When you mount an SMB share via Go > Connect to Server using the IP address, the underlying folder is what macOS registers as the target mount point.
Furthermore, the NAS name still appearing in your Finder sidebar despite being "turned off" is a result of mDNS/Bonjour broadcasting from the Synology NAS, which operates independently of whether AFP is disabled.
Finally, you cannot add the top level of the Synology shared folder directly to the Login Items folder, you can only add subfolders. For example, with the file share volume on my DS223j mounted, it cannot be added to the Login Items folder, though a subfolder on the volume can be added directly.
Instead of trying to add the IP address or the mounted volume directly, you need to create a short script or Shortcut, save it as an app, and run that app as a login item.
For example, this was posted by long-time member @Tesserax, and there are other options in that thread.
The script I use is the following. You would just need to replace the <> place holders with your NAS's IP address and user credentials.
mount volume "smb://<path to NAS>/<share name>" as user name "<NAS username>" with password "<NAS password>"
end try
Compile the above script as an application. To do so, from the Script Editor's menu bar, hold down the option key, and then, click on File > Save As... Change the File Format to: Application, and then, click on Save.
Then add this application to the user account's Login Items via System Settings > General > Login items & Extensions > Open at Login
Source thread: Unable to add a Synology NAS device to lo… - Apple Community
You could achieve the same result with the Shortcuts app.
Shortcuts User Guide for Mac - Apple Support
User level: Level 1 (15 points)
