I have comcast/RR with a linsky router and have no problems with running 3 or 4 puters at a time. Just my 2 cents worth. Does sound like a router problem though.
I understand, mine worked before it didn’t. ;)
Seriously. They said it would do that sometimes with the potential for both to stop working. (Their words,not mine)
At work, we had a modem failure, again RR, multiple routers, etc. After finally replacing the modem, the tech could not get us back online. After he left, I unplugged everything and rebooted back in sequence. Patience between starting devices, a couple of minutes anyway, some boot far faster than others.
Please ignore if you already have an answer, didn’t have time to read through the thread.