Think of the internal side of the SME as the LAN, and the external side as the internet.
If you dialin to the SME, the client will be on the LAN. Just like any computers connected to the internal interface.
A VPN connection creates a secure tunnel between two internet computers. In this case, that would be the client and the SME. The VPN connection lets the insecure internet client create a secure connection to the SME, and gives the client a LAN IP address. The remote client becomes a member of the LAN, even though it's physically on the internet side. To do this, the client PC needs to have its own internet connection already, which is why it won't help you in this specific situation..