If you can reach the router interface via your N900 means your DNS resolution is failing.
It can reach any host by it's IP address, but not by it's domain name.
The translation from www.google.de to the corresponding IP address of this host isn't working.

This is sadly beyond my knowledge. Usually you change /etc/resolv.conf with the IP address of your nameserver, which should be the router's ip address, but the N900 uses some other kind of resolving mechanism afaik.
So please wait for someone to respond here who knows about DNS resolution settings.