Had this issue last week and finally got it resolved last night. Took nearly a week to realize it was my Roku causing it.
Same router model as OP (nighthawk c7000v2), Xfinity internet, and Roku ultra (new in 2022) as well as Roku express (older, probably 2019). I have a 2nd ultra that connected with NO issue, and all other devices would connect to the network without issue. The moment I tried to connect the first Ultra or the Express, the whole system went down, required unplugging router, Xfinity to refresh the signal, etc.
Last night I did a chat with Roku support, and while they weren't clear about what caused this issue, other than to call it a "bug," they did have a fix that has worked.
On the infected device, go to: settings, system, advanced system settings, network connection reset.
They first had me connect to my mobile phone hotspot, which may not have been necessary but I did it with both devices. I then connected via my normal internet connection. After that it connected per usual, and hasn't acted up again.
A very strange bug. On the bright side Xfinity did find the line into my house was suboptimal and replaced it, so I guess some good came from it, but the past 7 days were VERY frustrating.
