I'm new at all this, is it not possible to do something like this.
If External IP Address resolves to eth0 IP range then you have a issue, in my case eth0 is internal network and eth1 is cable modem.
So let’s say my internal IP range is 10.0.1.x and my External IP Address in Sail is set to xxxxxx.dyndns.org and that resolves 10.0.1.x, issue found.