I can't connect to my new R6400 router, even though I get internet by connecting directly to modem

I figured out a solution, though I don't know what the actual techinical problem was. To get internet access, you need to tell Xfinity that you have a new network connected by going to xfinity.com/activate. I had done this several times with my computer tethered to the modem and no router connected. This was always successful, but when I hooked the router up, I couldn't connect (even though the router showed a white internet LED). I decided to hook everything up how I wanted it before I went to the activation link: router connected to modem and computer tethered to the router. Then I got on my laptop and went to xfinity.com/activate using my phone's hotspot. I went through the activation process one more time and then my router started working (ethernet and WiFi).