I had the same problem. It took almost 2 hours on the phone to figure out with Filemaker, Comcast (my ISP and router supplier), and Netgear (router mfr.). If you are properly set up for sharing the file on your computer, your router is forwarding port 5003 to your computer, then it could be the same problem I had if you are Mac based--a bad server.pem file. The file is located here: Applications>Filemaker Pro>(right click on) Filemaker Pro.app (choose Show Package Contents)>Contents>MacOS>server.pem. The size of the server.pem file on my machine was 0 KB, telling the Filemaker tech guy that it was a bad file. He sent me a new one and all is fine. New file is 8 KB.
Hope this helps.