I had a very similar problem. I ultimately solved it by having my entire system connected as I wanted it (i.e., the ethernet cable connected between the cable modem and the wifi router), with everything powered on, then called Comcast/Xfinity on my cell phone. I verbally spoke the option "no internet connection" when the robot asked me to describe my problem at which point the robot reset my entire system remotely. It sent me a text message saying it was starting the reset and then hung up on me. Ten minutes later I got a message that my system had been reset. At that point, I was able to go through Netgear's protocol of setting up my new Wifi router, which included updating the Wifi router's firmware. It was however, obvious as soon as Xfinity had reset my system that things were going to work.
Further comments
1. I spent many hours trying to troubleshoot this before hitting upon the relatively quick solution above.
2. I called Comcast/Xfinity twice before my successful call. For these first two calls I had said I need to "Activate my system". Each time I was connected to a human. The first human got my information then tried to transfer me to an activating agent at which point my call was dropped by Comcast. The second agent insisted that I did not need to activate, said she had been working for Comcast for 20 years, that she could not possibly be wrong, and hung up on me. So the trick was getting a robot, not a human.
3. Comcast documentation and human advice seem to convey that once the cable modem is working, it doesn't matter what is hooked up downstream. This is demonstrably false. I was able to hook up only one device at a time (my old wifi router, or my computer directly, or my new wifi router, or another computer). In order to swap these out, I needed to use my cell phone to call Comcast and have the robot reset my entire system. In the case of directly connecting a computer, there was a way to do this directly form the computer browser, but I couldn't figure out how to do this directly with a wifi router.
4. Before hitting on the correct solution, I tried a number of tricks, like setting a static IP to the router equivalent to the IP that Comcast had previously assigned my desktop computer. But I was not successful.
5. I was getting close to giving up and surrendering and buying an Xfinity Gateway. Xfinity definitely has a conflict of interest in providing advice or making it easy to connect your own gear.
6. It wasn't strictly true that I had "no internet connection" because I did have a connection through my desktop computer. I just couldn't figure out how to change that connection to the wifi router.
7. I was not able to update the Netgear Nighthawk router's firmware until after I had the system reset by Comcast.
8. Merely turning off the power to the cable modem does not work. It has to be reset remotely by Comcast.
9. After posting my original message in another thread, someone suggested a Factory Reset of my cable modem (using the button on the modem) would work equivalently to having Comcast reset my modem remotely. I did not try this, but I suspect it would not have worked for me, as there seems to be a phenomenon of Comcast knowing what I have plugged in downstream. Not sure how, maybe through the MAC address. So you might try the factory reset button. But it is probably just as easy to have Comcast do it remotely unless you don't have a way of phoning them with a cell phone or other phone.
