My comment isn’t really going to help, but ...
I have a client who was not able to get to her website *at all* during a sliding window of time via a particular ISP from her house. I could get to it from where I work using a different ISP. The ISP’s tech people could get to it. But from her house, during a 4-6 hour window of time in the morning, it wasn’t available - even on my computer. Other sites were.
We then determined that other neighbors using the same ISP in her neighborhood would experience the same thing with this particular website. Checked with the hosting company and nothing was amiss. Her ISP repeatedly insisted it was a problem with her house (I know, doesn’t make sense that others would have the problem) until I showed them that via an IP address the site was accessible but not via a domain name - ie it was a problem in the ISP’s network.
Of course, their tech support doesn’t deal with their own internal network problems, so the problem had to get bumped up to a higher level. A call to the public utility commission accomplished that wonderfully
All that is a long way of saying I’d be looking for a router problem too.
Matt