You need to have ports open in your firewall to get outside. 5003 is for communicating between FileMaker Pro and FileMaker Server
Here is a list of all ports for FileMaker
Best is you have a domain url www.yourdomain.com that you set to your static IP for your server.
Then your users will access your FileMaker WebDirect solution from www.yourdomain.com/fmi/webd/
I have already assinged the port 5003 to my router but to luck.
For WebDirect you need port 80 (http) or 443 (https) open on your firewall and forwarded in your router to your FMS box.