R7000 Nighthawk

You could be confusing the WAN and LAN connections. You NEED the WAN connection to get to the Internet. The LAN connection is what allows you to connect to the Router. On top of that you have the connection method. Wired or Wireless. If connected WIRELESSLY this will add to the confusion quite possibly.

First are you WIRED to the Router (appears you might be as you think you have a bad cable)? If so, and on a Windows PC open a CMD prompt and enter "IPCONFIG /ALL" without the quotes and press enter. On the output look for the Ethernet Adapter and then the IPv4 Gateway address.... usually 192.168.x.1 where x is 1 or 0. Open a browser and enter that address for the URL and press enter. You should get the sign-on prompt. If the IP Address is anything else please post it on a reply.

If you are WIRELESS, then the problem might be that the Modem is a Modem/Router Combo that Spectrum provides. IF IT IS, Spectrum MUST bridge the Modem and disable the Wireless signal output. If you are using the modem's wireless signal then that would explain you can't see the router.