darmasanthi
When behind a router, usually your sme would be in server only mode. You would then forward ports from the router to your sme server for each public service you wish to use eg mail, http, https etc.
It seems you may have a DMZ setup, so your situation is not as simple.
You have options in the console to configure the server and probably need to select fixed local IP.
You are asking for a howto, but there is none I'm aware of, probably best to consult the Smoothwall documentation or ask at a Smoothwall forum.