Probably the only true way of knowing is by setting up an EXTERNAL host somewhere on a VPS or maybe a reputable VPS provider. Then, on that provider, set up Uptime Kuma, or if you don’t want to go through that trouble and don’t mind a potential 10 minute gap in knowing, uptimerobot.com which checks every 5 minutes and sends an alert.

Once you do this, unless you have a Static IP, you will want to register with a DDNS provider so you can then tell the uptime service to ping your DDNS host which should echo back . If your internet is down, it won’t echo back and then it will trigger their alert. Of course, this won’t work if your IP changes, so staying on top of that is key unless you use a router which auto updates it which a lot do now days.

Or, if you use Cloudflare Tunnels, it can be configured to alert you when the tunnel is down or unhealthy (A.K.A. No internet or the server is rebooted).