You don't need to specify DNS on N900 if you use DHCP ('obtain IP adress automatically', I believe it's called in the menu) under the 'Advanced' tab. You'll need to specify on your router though, refer to your manual as to where.
In the first screenshot you can see where you can change radiotype on the router. AFAIK its not necessary on the N900 its only 802.11 b/g compatible anyway.
It would probably help if you could post make and model of your router.