There are two methods to connect to the main cpanel administration interface:
1) By ip address or domain using https and port 2087, example: https://www.mydomain.com:2087 or https://ip_address_of_the_vps:2087
2) By adding /whm at the end of domain or ip address, example: http://www.mydomain.com/whm or http://ip_of_the_vps/whm