Once you have created an HTTP check with your Datadog Agent, you might want to monitor it on your Datadog application
To build a Network monitor on an http check you need to:
1 - Create a network monitor: Monitor > New monitor
1bis - Select "Network check" monitor:
2 - Enter your network monitor setting:
Don't forget to select the threshold values for your monitor
3 - Configure the 4 - "Say what's happening" section to receive the correct notification.
Find more about this here
Note: Contrary to metric monitor it's not possible to get alerted after the endpoint is unavailable for X min. Instead you can only be alert after 5 max consecutive bad statuses. Unless a high timeout value is used in the agent config, if a site goes down this translates into 5 * ~15-20 seconds (agent collection period) i.e. 1min30 without data.