Did you create an A record
w154.white.fastwebserver.de pointing to your static IP of the router or the ISP config Box.
If you haven't created yet create one now and it might take upto 72 hrs for the DNS propagation.
ping w154.white.fastwebserver.de and you should get reply from the router or ISPconfig Box.
If you wish to use some other DNS server just fill in the IP address of the DNS server you wish to use or you could use your Internet service provider's DNS server IP instead.