Uptime-Kuma has push monitoring. Get the modem or another device connected to it to curl or wget the URL every minute then have Uptime-Kuma send a Discord or Slack or whatever notification to you. When the url becomes unavailable, you get a notification.

Host it on a VPS or you can use Fly.io for free.

Can you connect something like a raspberry pi to the modem and send a request to some server?

You could build a heartbeat app. So you send a request to the heartbeat app and just save the timestamp of the last request.

If the heartbeat app doesn't get a request for some time, you can assume the modem or e.g. raspberry is down.

Couple of options. If you have a VPS you could install something like Nagios and do passive updates. Set a stale threshold and have the stale check report as a warning.

I have multiple wifi sensors that boot, perform their function, then go into deep sleep for a set time. I set up a passive service type that changes the status to unknown with the message "data acquisition failure" if the stale threshold is hit.

My other setups all have a static ip so was never an issue. Looks like only way is to have a physical device connected to the modem. Was hoping to have ddns on modem and have a ping of some sort..