First, please check that API access in KeyHelp® is set to "Enabled" so that the API receives and responds to requests from WHMCS.
After that please log in to your WHMCS server via SSH and try to connect to your KeyHelp® server via "curl":
curl -vvv https://your_keyhelp_domain
If the call via "curl" is possible, you should also test the connection via PHP.
To do this, create a PHP file on your WHMCS server with the following content and then call the file via your browser.
$ch = curl_init("https://YOUR_KEYHELP_DOMAIN");
curl_setopt($ch, CURLOPT_VERBOSE, true);
$debug = fopen('php://temp', 'w+');
curl_setopt($ch, CURLOPT_STDERR, $debug);
$a = curl_getinfo($ch);
$b = stream_get_contents($debug);
In the third line the domain of your KeyHelp® server must be inserted.
The script returns the whole history of the connection setup from your WHMCS system to the KeyHelp® server. (verbose)
This is a good way to check at which point the connection breaks down. Most of the time, firewall settings on the WHMCS or KeyHelp® server are the reason why the connection between the two systems is not established.
We are also happy to help you via our contact form