You need at a minimum two things:
1) a static IP or static DNS address that points to your filemaker server from OUTSIDE of your network. This will NOT be a 192.168.x.x IP address. Most likely you will want to contact your internet service provider to have this added to your internet service for the office where the server sits. Static DNS can be had through services like DynDNS. A static IP address isn't really something you can "create" on your own.
Note that when you go to a site like "whatsmyip.org", it will show you your current WAN (outside) IP address, not your local (LAN, 192.168.x.x) IP address. This WAN address changes each time your router's DHCP lease is renewed, thus it is NOT "static". Having your ISP assign you a static IP address guarantees that WAN address is assigned to your office.
2) You will need to configure your router so that incoming requests (from outside) on port 5003 get forwarded to your filemaker server's LOCAL IP address (this IS the 192.168.x.x address). This is configured in the router. Sometimes the router is part of the modem, sometimes it's separate hardware. Usually you can google your router model number & " port forwarding", and find guides on how to do this.
The end result is that someone with filemaker go in the field, should be able to add a favorite host for your static IP address, and be able to open the file.