A simpler solution can be implemented using the UDP Broadcast Relay package. This solution does not need IGMP Proxy and it does also not need AVAHI Daemon.
Step 1 Enable Multicast Traffic (replaces Step 1 in the solutoin by @smol )
- create an instance
- select the interfaces for the networks that should be able to speak SSDP to each other
- set Spoof Source to Keep original
- set Destination UDP port to 1900
- set Multicast group to 239.255.255.250
Step 2: Configure Firewall Rules:
see Step 2 in the solution above by @smol