masotsha
You don't need portforwarding, if I assume your situation correctly.
From the admin console run the "Configure this server" option.
Step through all the screens without changing anything, & towards the last few screens you see a page where you specifiy a server as the gateway for Internet access.
So if you have a main sme server on say 192.168.1.1, then a second sme server behind it on 192.168.1.10, then point the second server at 192.168.1.1 for Internet access.