Ubuntu wont see internet with DHCP disabled on router

we recently brought a NAS box for our home network.
We wanted to access this over the internet, however our internet setup is such that we have a box on the roof, which our ISP controlls, and a router downstairs. We had to reconfigure this, so that our router only acted as a switch/access point, and the box on the roof is the DHCP server and also sorts out the NAT. The ethernet cable that comes in from the box on the roof has to be put in a LAN slot of our router (1 of four which would normally be connected to computers)
Everything seemed to be working fine, our router is the same IP as the box on the roof, and is effectively invisible.
I am the only person using ubuntu in my house, and everyone elses computer works fine. However i cannot access the internet.

When i use another computer (win 7/xp) and type in 192.168.1.1, i get the interface of the box on the roof, which is what should happen. However on my laptop with ubuntu, i get the interface of the router downstairs. I can use our intranet problem free, but the internet just is not there on my laptop.
I have plugged my laptop in to the router downstairs, restarted everything and checked connections to no effect.
Any ideas?

thanks

jake