Nighthawk RAX45 wont connect to internet

I really need some help.

I followed the app instrucstions, but I can't get past the "cable" connection error which is right before the "connected to internet phase". It's like the nighthawk doesn't work with the cable or the modem. I have a CM600 modem and checked with ISP Comcast to make sure it's up to date. I also swapped out the cable it came with a for different cat5e cable that I was using previously. It just won't connect. So I plugged in my old router, that worked just fine, I piggy backed the nighthawk to my old router with the same cat5e cable it came with and I got internet then. So I configured it and installed the software update. Now both modem and router and up to date. I unplugged it all and set up the nighthawk again connected to my modem, after power cycling the modem, and still no internet. I'm at a loss, it works piggy backed off my old router but won't work directly with my modem and this is after resetting the modem. I did it a couple times. Please help, been eyeing this router for months and finally purchased.