@ChrisC84 Different Deco model, but on mine, once I found the IP address (ran arp -a on my Windows computer to get a list and just tried to connect to each), I accessed the My Cloud from the browser and then it showed up in the app. 

 After that, I used the "Address Reservation" so the My Cloud will always have the same IP address.

@RichT 

Thanks so much for your quick reply. I have done exactly that and it is none of them! I can't quite figure out why. I am starting to think there is some other setting on the WD itself, and I suspect I have to somehow directly connect to that to have a look - but god knows how.

@ChrisC84 On mine, I did restart the WD My Cloud just before looking for the IP, figuring it would need to connect at that point. Might be worth a shot.