To access your cPanel / WHM / Webmail if you are behind a proxy that is blocking access at work, school or university etc. you can use the following subdomains:
https://cpanel.yourdomain.com
https://whm.yourdomain.com
https://webmail.yourdomain.com
These will allow you to bypass the proxy as the traffic will go over the standard web port 80 as opposed to a custom port.