Hi, this is exactly the way I have setup all similar situations that I have dealt with. That way you keep all your resources within your network and just expose what is required. If you note my original response you will see recommended just port forwarding the 2nd PubIP to the other server. I suspect thought that the Fritz does not let you DSTNAT any IP other than the one it is aware of, ie the one via the pppoe client.
Cyril