Nighthawk R7000 with CM500 Modem on ComCast - internet light always off

Why they sent you here I can't understand? I would have called back to talk to a different person or asked to speak to a supervisor.

One question or I guess questions? You've got a laptop, right? It has a cable that you used to connect to both the modem and the router, correct? It works fine to the modem but not to the router, correct? Do you have a DIFFERENT cable that connects to the modem to the router? If so, it is possible THAT cable went bad? Do you have another to try? If not, use the laptop one between the modem and router. Do the lights work properly then? If so, the cable is bad.

I also assume you've used the proper bring up method for the modem and router when changing connections. In all cases you must power OFF all items, modem, router, and PC (or any devices connected to the router). Then one by one, power them up and wait for them to fully initialize before turning on the next device, modem first, then the router and then the PC. Those devices store the MAC address of the item they expect to connect too. If not found they don't connect, so you can't 'hot swap' cables around and expect them to work.