Firewalld, if included in RHEL 7, is a replacement for iptables and is being included as default. Some benefits include not needing to restart the firewall when changes are made, which means your system maintains its firewall during rule modification, and current connections are not lost.
Anyhow, if you are not willing to migrate to Firewalld and want to use iptables as default, follow these steps:
Install iptables service:
yum install -y iptables-services
Set iptables as default:
systemctl mask firewalld
systemctl enable iptables
Stop firewalld and start iptables:
systemctl stop firewalld
systemctl start iptables