Minim Server not found due to firewall on synology

[MusikRBA]

I am using Minim Server 2 (2.0.12) on a Synology DS918+
In the synology firewall I have port 9790 and 9791 on TCP and 1900/UDP open for the MinimServer UPnP media server application.

As a client, I am using the Naim App for a NDX and MusoQb2. MinimWatch is running on an iMac.

When I start the NaimApp it won't find the MinimServer with the firewall ON.
When I switch the firewall OFF, it'll connect and everything is working fine.
When I subsequently switch the firewall ON again, the active connection still works. So I can browse Albums, play music and use the MinimWatch.

So the Synology firewall is blocking the initial connect or search on the LAN.

Any idea which port I am missing.

Any idea on how to debig this?

The support from this forumg is very much appreciated.