Don’t know if you have an answer yet, but my first question is who is your ISP?
I had an issue with RR last year when I tried to have ore than one computer running off of our cable. We have multiples at work, didn’t see where it would be an issue.
They told me a home connection would only support one computer at a time. This after little brother went to school and brought home every type and brand or router and splitters they had.
Switched to AT&T wireless modem.
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.
Comcast is our ISP. We have had no problems with two computers and an XBox ~ it was when the third computer was added (and one child downloaded stuff) that the problem started.