DHCP is really intended for a large network, where devices come and go regularly.
In a typical home environment, that’s not so likely, and especially not for fixed infrastructure like a NAS.
The approach I take is to leave all devices on DHCP, and let the DHCP server in my router allocate them an IP address within the DHCP address range, but, once allocated, I use the router’s control panel to set an ‘infinite timeout’ for this IP; it means that the DHCP-allocated IP address will always be allocated to that device, and should never change. Different routers might use a different name for this, e.g. ‘always use this address’ is the same thing. [edit: Joerg_A’s ‘static lease’ is the same thing]
This saves me having to look up all the information required by the MyCloud Static IP dialogue (which should be available using ipconfig, I think). I’m a bit lazy…
Some would argue that this reduces the range of available IP address for DHCP, but I don’t need that many IP addresses in a home network. If I were running a public-accessed network (e,g, a pub, bar or restaurant), I might think a bit harder about it.
Setting the IP address to static at the router should also work, provided the DHCP server is smart enough to know that when a client device requests a DHCP address (e.g. leave the MyCloud as DHCP), it recognises that the requesting MAC address is allocated a static IP address, and returns that address in the DHCP IP transaction.