Please draw us a sketch of your network when you connect the PC and when you connect the router. 

Or let me make it simpler:

When connected with the PC, show us the output of ipconfig. I am trying to establish if the IP assigned dynamically and what gateway you are assigned. You see, if the IDU is already acting as a router, cascading your router to it needs some thinking. You need to configure a static IP on your router's WAN interface (because some routers hate working with DHCP on their WAN). Remember you'll be having two level of NAT once you cascade so ensure that the IP subnet on your WAN is NOT the same class as the one your router dishes out via DHCP. If they are the same class/subnet, then there is no way your router will work!

With that, I see no point why it shouldn't work. I can bet 3 beers on it working that way!