cPanel/WHM not working after fresh installation

Issue :

After a fresh installation on a CentOS server and following all the standard steps, it is not connecting to WHM on port 2087.

Fix :

CentOS has default iptables (firewall) rules that block access to cPanel ports when out of the box.
You can flush them using the following command :

iptables -F

Also, remove them from /etc/sysconfig/iptables to prevent them from re-appearing on a system reboot or service restart.

This should fix the issue!

Source : cPanel Forum