I'm not sure why you have 2 routers on the same network. Is this something to do with wireless?

Are you saying you tried both the Zyxel and the Netgear at different times, or are they always on the same network with your Zyxel as the modem and the Netgear as the router or the other way around?

If they are on the same network at the same time and you have some reason for this, the Zyxel might have to forward to the Netgear (or the Netgear to the Zyxel), rather than the Dreambox, but this is a big might.

Could you describe your network in a bit more detail?

"My Zyxel is at IP etc. and connects directly to my ISP.
My Netgear is at IP etc. and does this or that.
My Dreambox is at IP etc."

How have you checked and found the port not open?

It would also help if you could do the following:

1.) Telnet to your Dreambox, type ifconfig -a and post the results.
2.) Open a DOS prompt on a PC on your network, type ipconfig /all and post the results.

Please edit out any MAC/Physical Addresses, host names, and DNS servers.

Thanks!