No internet on R8500 when pc connected to CM1200

Hello all,

I recently purchased a CM1200 modem and R8500 router in order to upgrade to Gigablast by Cox.  Currently testing setup and it works flawlessly as long as my pc is hardwired to the R8500.  But when I connect the pc to one of the other ports on CM1200, the R8500 loses connectivity to the internet and the pc encounters slow internet speeds (20Mbps).  I need  pc and R8500 connected to CM1200 since router will be in another room in the future.  Any help would be greatly appreciated.

Thanks,

Jay