C6230 help needed

I have a neighbor who replaced her Xfinity modem router combo unit with a C6230. Apparently before the switch out they were having issues that sound like circuit flapping, and I’ve advised them to pursue that with Xfinity. But, in the course of setting up the C6230, and coming back to see if there was anything on the modem or router side that would indicate what is happening, I found that I’m unable to log into the C6230. I downloaded the Nighthawk app per instructions, and went through the login via default log/password , no go. I reset the modem via the seven seconds with a paperclip method, still no go, it says the login/password combo is invalid. There are some times when the Nighthawk app cannot find the C6230.