In your screenshot you have 802.11b/g/n which is OK, no need to change. Do you have any devices other than N900 and are they able to use the wlan without any problems? If so then your router should be configured OK. As unsane said it's a DNS problem on your N900. On the N900 go edit your connection. Click "Next" until you get to the last screen then click "Advanced". Under the "IP Addresses" tab select "Auto-retrieve IP address" and select "Auto-retrieve DNS", then "Save" and "Finish".