JW333 wrote

I rather naively thought I would then just disconnect and reconnect to my primary M9 and it would appear on the network but 2 attempts at this have failed.

@JW333 

Just to make sure, you are not connecting WD NAS to Deco M9 with USB cable, right? You must be using Ethernet cable.

I am also assuming you rebooted WD NAS, after you disconnected it from virginmedia router, and did it hard way: power off (shutdown) NAS, unplug it from electrical socket, plug it back, connect Ethernet cable to NAS and M9+, power NAS on and wait till it boots.

When you connect WD NAS to Deco M9+, do you see it appear as device under Main Deco, in Deco app? Device name may be unusual, not necessarily WD or MyWorldBook, but it would be possible to figure that this is the NAS. If you see it in Deco app, you'll also see its new IP address assigned to it by Deco mesh.

If you don't see MyWorldBook in Deco app, how about lights on NAS, what do they say? I am not familiar with that model but usually NAS systems have a least rudimentary lights based status reporting described in User Manual, and if NAS has LCD screen - even better, more info could be found there.