SymptomsIf NAT is configured incorrectly, it is impossible to access web server from the CP server via external IP.
ResolutionPossible solution for Linux.
Add the following rule to the iptable:
iptables -t nat -A OUTPUT -p tcp -d <web_server_external_IP> -j DNAT --to <web_server_internal_IP>
Also, add this rule to /etc/rc.local so it keeps working after server reboot.