Cant login to router and no internet on a N600 WNDR3700v5

I havejust purchased a CM400 cable modem to acess the internet through Comcast and an existing N600 WMDR 3400v3 router. Previously I used a Zoom cable modem connected to the router via the ethernet cable. The router then connect to two computers via LAN and several wireless devices. This past week my internet conection started dropping out regularly and Comcast said they no loner supported the Zoom modem so I upgraded to the CM400. I installed the CM400 in place of the Zoom modem and had it activated by Comcast. 

I now find that if I use the moedm ethernet port to connect the router, I get no internet service. However, if I connect  the ethernet port from the router directly to one computers ethernet port, I get internet serivce. What really confounds me, though, is that if I connect my router directly to the ethernet port on the Comcast telephone interace modem, my wireless service works fine. It is only when I connect the ethernet directly from the cable modem to the router that I get no service.

Any suggestions to restore the connectivity would be greatly appreciated.