This may actually be port related, can you check the router configuration and ensure that is has the following ports forwarded to your HDX IP address:
TCP 1720, 3230-3243
You may also want to try ensuring Fixed-Ports is enabled on your HDX by navigating to:
System > Admin Settings > Network > IP > Firewall
Check the box for fixed ports, leaving the starting port as 3230 for both fields.
With these steps performed, do you still have this issue?