Here are my speedtest results.
Since your players can connect, you don’t have port problems.
To open ports, open your firewall settings, click on the Exceptions tab, ensure “Display a Notification when Windows Firewall blocks a program” is checked, click OK and start Freelancer. You should get a warning and should be able to check “Always allow”. Start FLServer.exe and you should get another warning and select “Always allow”.
The other way (not so reliable) is to add Freelancer.exe and FLServer.exe to the programs list in the Exceptions tab. (Click “Add Program” and browse to find Feelancer.exe and FLServer.exe).
You can open the ports (click “Add Port” and give it the port number (2300, 2302, 2303, 2304) and a name eg “UDP 2300” and check UDP) but I found this is not reliable either.
If you have no success or if Freelancer is in the Exceptions list then delete all of the Freelancer and FLServer entries and start both of them, one at a time, you should then get the warning messages and be able to allow them.
Important: “Microsoft DirectPlay8 Server” must be in the Firewall Exceptions list too, or your players will not be able to connect.