When trying to connect, the app displays a message:
Connection fail. Check your network settings and try again.
The solution to this is that next to port 5000 and 5001, the port 6690 also has to be opened to the Synology NAS. If this port is not opened, the Windows app will fail.
The Android and iOS apps will work with just port 5000 (or 5001 if SSL is selected and the certificate is valid).