Most uptime services don’t work well on free hosting due to the security system.
Also, because of the hits limit, you probably don’t want an uptime tool pining a page that has a lot of assets on it.
A better way to do it is to set the uptime system to use the IP address your website is on (found in the client area) instead. If the server is down, it will timeout. If the server is up, it will respond with an empty response (You may have to change the default configuration for the system you are using to accept an empty response as “up”).