Luckily the port forwarding on the Zyxel is rather easy. If you select Network Setting on the left hand side menu, then select NAT you will notice that the first section you are in is port forwarding. Select New Rule, and you will see the Port Forwarding rule box to setup and write the required rules. You need to make sure the rule is active, give it a service name, and bind it to the L2TP interface that you set up the dedicated IP to. Then enter your Start and Ending Port (3389 etc), enter the Server IP Address (This is the internal IP which you would like to forward the traffic to as well as the protocol. Once you have saved this, the forwarding rules should be set.
If you would like to test the opened ports on your dedicated IP, you can run an external NMAP on your dedicated static IP address and make sure that the ports are open on the routers receiving side.
Hope this helps (",)