Change cPanel root password for Amazon EC2

I was configuring cPanel in a Amazon EC2 CentOS server. I was able to reach the login screen of WHM by using https://ipaddress:2087. But I can't enter inside it. I tried all my root and other user passwords.

Is there any different configurations like "/usr/libexec/webmin/changepass.pl /etc/webmin root password" of Webmin, specific for cPanel?

Or is there any recommended method to install cPanel? I am ready to start other server and perform as CentOS needs fresh server. Also I have added the Elastic IP.