You'd normally do that from the NAS itself, so if there's a console program which you can install on your computer, use that.
If not, then assuming it's wired up to the network, it will have been allocated an IP address from the router. The Draytek's configure themselves as 192.168.1.1, so open up a browser with that address, and a pop up window will appear. The default username is admin with no password. You can then see what IP address has been allocated to the NAS. Open up another browser for that IP address (some need X.X.X.X:80 or :8080 to use), and then navigate around to find the setting to change to a static IP address.
No mapping of ports will be needed. You can configure the Draytek to not use a range of IP addresses, so it doesn't allocate the NAS one.