Monitor ports, DNS records and hosts with no web page
Check that a port accepts connections, that a DNS record still points where it should, and that a router or appliance answers ping.
Why it works this way
Plenty of what keeps a product running never serves a web page: a database, a message broker, the DNS record in front of your API, the router in front of the office. An HTTP check sees none of them directly.
A port monitor opens a TCP connection, with TLS if the service expects it, and passes when the connection is accepted. A DNS monitor resolves A, AAAA, CNAME, MX, NS, TXT, SRV, CAA, SOA or PTR records and can require a particular answer, which catches a record that still resolves but now points somewhere wrong. Point it at a public resolver to see what your users see while a change propagates.
A ping monitor sends one ICMP echo, for devices with nothing else to connect to. Many hosts drop ICMP on purpose, so wherever there is a port to open or a URL to fetch, those tell you more about whether the service works.
What it costs
Those capabilities and everything they depend on come to $5/mo.
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Build the plan around it.
Add what you need, drop what you do not, and read the total before you buy.