Short of paying for a service (which somewhat goes against the grain of trying to host all your own stuff), the closest I can come up with is relying on a service outside your network that has access to your network (via a tunnel/vpn).
Given a lot of my own networking set-up (DNS/Domains/Tunnels etc) are already managed via Cloudflare, I'm thinking that using some compute at that layer to provide a monitoring service. Probably something to throw next at my new LLM developer...