I’m curious to see what others use, but I’ve been using Uptime Kuma for this purpose at several different sites I manage. I run Uptime Kuma on a VPS and locally on site. I have the local instance monitoring the router, gateway, DNS, and several other internal and external devices. I also have the local instance do a “push” check-in on my VPS instance. This gives me a pretty holistic view of things.