This could be a DNS problem, make sure your server hostname has an A record set correctly to the server IP address to get it resolved. Otherwise, try accessing the root WHM panel via https://IPADDRESS:2087 where replace IPADDRESS with your server main IP and bypass the SSL warning shows in the browser.
It's definitely a DNS issue as I am not able to reach that hostname (which I redacted for security reasons). You'll want to check the DNS related to that name to get the issue resolved.