The port you need to forward is 5003 rather than 80. Give that a try.
Thank you for your post.
192.168.xxx.xxx is a local IP address; not your main IP address. On the machine hosting the files, launch a browser and go to whatismyip.com. This will display your main IP address. Then, when you are outside of your local area network, use the external IP Address rather than the local IP address.
Also, as "Troy Meyers" pointed out (Thank you!), FileMaker Pro shares files over port 5003.
Troy, thanks!!! Changed port and FMGo worked over 3G immediately albeit slow. I still need to check a wifi connection from another site, but I dont see why that won't work. I will keep you posted.
Provided your WiFi test location doesn't block 5003, it ought to. I'm don't know if Starbucks blocks anything, but I haven't heard anyone make mention of such a problem. Not all WiFi hosts are wide open.