Uptimes monitoring options are described as
Here are short details about monitor types:
http(s): that's perfect for website monitoring. The service regularly sends requests (which are the same as if a visitor is browsing your website) to the URL and decides if it is up or down depending on the HTTP statuses returned from the website (200-success, 404-not found, etc.)
ping: this is good for monitoring a server. Ping (ICMP) requests are sent and up/down status is decided according to the "if responses are received or not". Ping is not a good fit for monitoring websites as a website (its IP) can respond to ping requests while it is down (which means that the site is down but the server hosting the site is up)
keyword: checks if a keyword exists or not exists in a web page
port: good for monitoring services like smtp, dns, pop as all these services run from a specific port and Uptime Robot decides their statuses if they respond to the requests or not.
This shows that the ports the monitor are expected to have the relevant service.
I would just use ping and allow ICMP requests through your router. Often these are allowed through by default, sometimes they need any other port opened to the Pi and sometimes they have a specific entry for forwarding but that depends on you kit.
You could create a small monitoring Python script that checks temp, SD / Disk space and memory and if any are low use Flask to put an error on a web page for monitoring...