Your internet connection is Cable or DSL, the Modem | Router that connects has a Web IP Address.
This address can be found at http://whatismyIPAddress.com
IF it is a Modem, only one computer can be connected, unlikely.
IF it is a Router, it does not know which computer to send the FMP IWP request to.
The Local IP Address for the FMP hosting computer is listed in FMP's IWP setup window.
The Port selected in the IWP setup defaults to Port 80 (same as a website server).
591 is the port set aside for FMP IWP traffic - you can set this as the port in FMP's IWP setup.
You must Forward the IWP requests to the correct computer on your local network.
http://portforward.com has instructions for hundreds of routers - knowing your make and model is required.
If your hosting computer's Local IP Address is 192.168.1.101 your router's Local IP Address is 192.168.1.1
Setting up the Port Forwarding requires a wired connection to the router.
There is a default UserName and Password for accessing the router setup, AND it may have been changed.
Taping the IP Address, router Username, and router Password to the router is a good idea.
If your router has port forwarding set correctly and your Web IP Address is 188.8.131.52
AND you are using Port 80 outside access in a browser would be http://184.108.40.206 OR 220.127.116.11
OR if you are using Port 591 outside access in a browser would be http://18.104.22.168:591 OR 22.214.171.124:591
The question has been asked before. You can see other explanations here.