I assume that most Internet provider these days provide a modem/router to which multiple devices could be connected. In these cases, the local network is using a private IP address range that is not accessible from the Internet, and the router performs network address translation for outgoing connections.
If this is the case, you have to configure the modem/router to forward incoming connections to port 22 (or other) to your SFTP servers's local IP address. This is somewhat beyond the scope of Rebex SFTP support, but there is plenty of information on the web about this. Try this search, for example.