Getting the WAN address for iptables command

[vgaetera]

DMZ and no access to ports on public ip?

It looks like you are solving the problem from the wrong side, because according to those port forwardings you should set up a VPN. And if you want to access a service by the same name from LAN and WAN without VPN, just rebind it to LAN IP-address on your LAN DNS-server.