See if you can get it to work with WEP at least. Then try your different WPA options in the router. Aside from that I don't know what to suggest.
So, is it a bug that the N900 will not allow you to disable WPS? My laptop connects to an open network with no questions, while the N900 asks for PIN or or to push a button on the router. If I'm unable to provide either, can I not connect to this network?