CentOS now comes with firewalld instead of iptables at the installation of your Dedicated Server.

You can remove the firewalld service and install iptables to manage your firewall rules.

First you need to stop and remove firewalld

systemctl stop firewalld
systemctl disable firewalld
yum remove firewalld

Now you can install iptables

yum install iptables-services -y

Enable iptables at the boot

systemctl enable iptables

Start iptables

systemctl start iptables