Once you configure NTPD service on your RHEL7 linux you will need to allow traffic through the firewall on port 123. Te following linux command will enable all public traffic on UDP port 123:
# firewall-cmd --zone=public --add-port=123/udp --permanent success
To apply the above changes reload your firewall:
# firewall-cmd --reload success
Use iptables-save command to confirm that UDP port 123 is now accessible:
# iptables-save | grep 123 -A IN_public_allow -p udp -m udp --dport 123 -m conntrack --ctstate NEW -j ACCEPT
Alternatively you can use graphical tool
firewall-config to allow NTP port 123 on your Linux system.