So I've just tried to do this in the router config. I've entered the MAC address for the NAS into the Static DHCP Entry screen. However, there is an IPv4 field as well, and when I try to enter only the MAC address and leave the IPv4 field blank, I get the error message:

"Please input available bind IP address"

Any idea what I should do with that? The router tells me the current DHCP assigned IPv4 address, which I could enter here, but I thought the idea of doing things this way was to not assign a static IP...?

HC: 861v4/ID40/LPS, DSP5000, DSP5000VC, DSP3100, SW1600, HD621, Oppo BDP-105, Sony VPL-HW55ES
2-Channel: Oppo BDP-105, TEAC VRDS-7/Trichord Clock 3/PSU, 551, Quad 303, Harbeth Compact 7ES-3, Kimber cables, Russ Andrews mains and supports.
Previous Meridian: 5000C, 5200SL, 7200.2, 6000, 562V.2, 568.2mm, 800v4, 808.2i, 818v2/v3, 861v8, 502