Thank you for your post.
Ports 5003 and 443 (999 in your case) are used to upload files to FileMaker Server. A simple "Connection Failed" message when attempting to upload is usually an issue with Port 443 (999). Could be that the FileMaker Server machine is using or refusing connection on Port 443 (999). This could be due to Port Forwarding issue, or Third-Party software on the Server Machine itself. You might try uploading a File to Server with FileMaker Pro on the Server machine itself. If that works, it's an inbound issue. Otherwise, most likely something blocking or using the port .
I hope this information is of use!
Thanks TSPigeon, exactly as I had suspected. I know the port is open on the server - I can upload when on the local network. Will get the router settings re-checked