On the ZyXEL: Go to (click) SUA/NAT (below Advanced). On the first line (1) check check box active, name RunUO (arbitrary name), start port 2593, end port 2593 and click Apply. Maybe you need to restart the unit.
Let's see how far we get with this (ports forwarded on the ZyXEL).
When restarted, try this again:
http://www.canyouseeme.org/