Your internal IP address is usually either 10.0.x.x or 192.168.x.xx THESE are LOCAL IP addresses.
http://whatismyipaddress.com shows what is your INTERNET address, this address is owned by your ISP provider and shared to you temporarily (or sorta permanently)
If you what is my IP address is 18.104.22.168
AND you have told your router to forward port requests for PORT 591 to your computer at 10.0.0.12 which hoste the Filemaker database
You will not get the response you want.
Thanks! So I may have to do some port mapping? I don't know what that is but suspect that's the solution. I'll see what I can do.