Hi,

As to the Netgear router, the PC and Dreambox are plugged into it because the Zyxel modem simply has no (ethernet?) ports to plug them in directly.

I changed the forwarding address in the modem to the IP address of the router. Port 12000 still remains closed.

As you proposed I had a closer look at the firewall in the modem. I briefly inactivated the firewall in the modem to see what happens. It results in port 12000 being open. I activated the firewall again and added a line to the rules of the firewall:

Packet direction: WAN to LAN
Source IP: any
Destination: <IP address router>
Service: Any(UDP)
Action: Permit

This results in port 12000 being open and (at least) 1 client connected.

Two new questions occur to me:
1. Could you please comment if the additional line I added in the firewall of the modem is safe? Is there a "security hazard" in doing so?
2. At "Service" one can choose Any(UDP) or Any(TCP). Does it matter? (I chose Any(UDP)).

Thanks.