nighthawk app can't modify settings when nighthawk cax80 modem is offline

My cable modem (with wifi router) CAX80 can connect to the internet over the coaxial cable. The Xfinity tech came over and set up the connection.

I decided to take the modem offline in order to change the settings on the WiFi router. I prefer changing the WiFi settings before turning the WiFi on.

I have the nighthawk app installed on my Pixel 6 - I was able to log on to the router using an ethernet cable - however, when i start changing the password and security questions, it will not save the new data. The error message says that the routerlogin.net/..... site is unreachable.

I also tried logging into the router using my PC, with the same results. I can get to the page for changing the password. Saving the information does not work.

As a last resort, I logged onto the router using WiFi. It asked for the serial number, but on the next page, I got an error: http://192.168.1.1/goform/PwdRecCheckserial?id=.... is unreachable.

Do I really need the connection to the internet in order to access/modify the router settings ?

Thanks.