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
systemctl start iptables