It’s probably worth checking with the ISP then (if you haven’t already) to confirm that they can’t see any issues from their side. It can depend on the technology used to deliver the service. Fibre services are usually pretty reliable, but we see a lot of issues here in Australia with drop outs in some areas that are using VDSL over copper phone lines, and the ISP has systems that can help diagnose that sort of issue. Your router may also have some logs that could help determining what the issue is.