It may have something to do with the setup on the madem and router. If your modem is also a router (most likely is, if you can connect a cable to it and get internet) it is probably set up to route the traffic to the internet on its own. That means that there is probably a DHCP server running on it. If so, you will need to check the setup on both the modem and N900 to see whats going on.
In a modem to router setup you will have to select which of the two will be doing the routing. I would suggest the N900 (newer and surely more capable), then enter the setup of the modem turn off the DHCP, firewall, NAT and generally all the functions also offered by the N900. Allowing both devices to do the jobs in parallel will cause you many problems. All you want the modem to do is give access to the internet and pretty much nothing else. I mean, you probably paid a pretty penny for the N900 why not let it do it's thang!