Configuring Netgear Nighthawk R8000 Wifi Router

Are you powering off the modem when you switch back and forth between direct to PC and router for testing?

+1 for power cycle each time you change what the modem is plugged into for ethernet. If that doesn't work, check your IPv6 settings. Last, do you have another router to test with?

Yup, cycling power completely as part of the test.

I installed the old modem, had Cox reinitialize it, then tested the Wifi router.  Worked like a champ (because it received an IP address from the gateway modem vice Cox).

For clarity sake, what model was the old modem and what model is new modem? What do you see when you sign into the app to reset the modem as per here? That will show what is currently provisioned on the account.

Also, concerning the R8000, what do you see for your WAN/Internet IP? Only expose the first number of the IP for privacy reasons. Is it 0? 172?

Finally, can you check the firmware on the router?

Old:  Netgear CG3000D (DOCSIS 3.0); new:  Netgear CM1100 (DOCSIS 3.1).

App states it sense cable is correctly connected; prompted to provide a static IP address (which I discovered through research means it's failing to receive a DHCP address).

No means of determining WAN/Internet IP as app will not allow anything further without an IP address.  To answer a question you didn't ask, I can ping* the internal side of the R8000 because I can connect to the SSID using the key included on the label of the R8000.  *The ping returns ten successful attempts then times out.**

Cannot upgrade firmware as app will not allow anything further without an IP address.

**maybe it is time to consider moving on to a mesh WiFi network ..........................