What should I look for when I enter the router interface?

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.

Look at these:
N900: Cannot connect on Wifi at Home
Whenever I connect to ANY WIFI connection, my web and data services stop working

Ok people I successfully managed to enter the router security panel with the adress http://192.168.1.1 what should I do now?

Ok I don't know what happened, I didn't changed anything in the router interface panel, and suddenly after I closed the panel my N900 magically connected to the internet...

I think the problem is finally solved...

Thx for the support guys.

Cyaz

FYI: Accessing the router interface was only a test if your general network connectivity is working.

Glad it works now. Must been some kind of random router/n900 fubar.