The problem is due to the system restricting access over TLS 1.0 when the .NET app should be using a newer version (i.e. TLS 1.1, 1.2, etc). The following steps should resolve the issue:
Step 1 - Access the CHTAgent log in the Windows management event logs associated to the affected instances to check status. This will show the actions being taken by the agent and flag any errors. This particular issue will show "Failed to register with server: 0" as an error in these logs.
Step 2 - In order to force it to use the latest TLS version, additional registry keys will need to be added.
To do so, run Regedit and navigate to the paths:
In EACH directory add the following two keys and value pairs:
You may need to add the same keys if you have other versions of .Net installed.
Step 3 - Once added, go back into Services.mmc and restart the CloudHeath Agent Service.
Step 4 - Confirm registration is successful in the Event View mentioned previously.
After these steps are complete you should see agent status update and check in within 15 minutes.