You can set your router to provide the nas as a dhcp client but still manually configure an IP address.
In your routers config, you should be able to set the NAS's IP address manually. It will still be dhcp, i.e. it will still get its ip from the router, but the router will recognize the NAS by its MAC address and assign it the same IP every time.
I do this all the time. All my clients are standard dhcp. 192.168.0.2 +1, +1, etc.
All my devices that provide services: NAS, Music server, network printers, managable switches, etc all are manually assigned, dhcp devices counting down from 192.168.0.250.
So everything on the high end is fixed and everything on the low end is dynamic.
by the way, the mac address of the nas is physically on the device, usually a sticker. If you PM me I can send you a screen shot of the router screens.