Simple service to check infrastructure status
This is conventionally called a watchdog, deadman, or health check.  Bruce's comment is good, including the part about a raspberry pi.   You could also spin up something simple like monit in a container someplace like GCE or AWS.
Other than setting up an external service or server, there are also a bunch of phone apps that will do the pinging and/or http fetch and HTML parsing for you, right from your phone, providing you with a notification and/or audio alert when a condition isn't met. Since you're a one-man shop, using your own phone isn't substantially worse in terms of being a single point of failure, though you'll want to have a migration plan for when and if you do grow.
The thing I would probably do myself is keep the more complex monit configs on-site, summarize those on a simpler monit config from a stable IP at GCE, with a simple config on my (dynamic IP) phone watching the GCE monit.  That lets firewall rules keep things relatively private other than that summary query from the phone, and when you add a new team member they just need to add that phone app with the one query.  If you're comfortable with leaving the firewall rules more open, then you could skip the GCE step and just have your phone hit a summary statistic on your own server directly.
Back in the depths of time I used to use SMTP mail for this sort of thing a lot; these days I'd prefer the phone app for a more timely notification.