Firewall rules / port forwards for Goodwe UDP based integration

Hi,

I am trying to get the Goodwe non-HACS integration to work through a firewall into a different network.

The Goodwe Inverter is on one network while Home Assistant is on another. An additional Raspberry Pi has interfaces in both networks.

Now I don´t get the port forwarding to work. I understood that the inverter is called on UDP port 8899 (sometimes 48899). This port is is opened with a firehol / iptables rule
dnat4 to 172.16.2.122 proto udp dport 8899

The usual pyhthon test script works from the RPi itself but not from any machine on the seperate network.
I did try UDP broadcast forwarding but it never gets triggered by any packet.

Reading through many posts about the Goodwe protocol I never understood how the communication back is supposed to work. Can somebody help me?