*** EDIT ***
Thanks Mariano. Got it working in the end.
In a nutshell, for anyone else trying this;
Enable SSH on ReadyNAS https://kb.netgear.com/30068/ReadyNAS-OS-6-SSH-access-support-and-configuration-guides
Use PuTTy to access Digital Ocean server and type
in the editor, copy-paste in the public key text used by ReadyNAS, appending this at the end of the /root/.ssh/authorized_keys file (be careful to not overwrite any keys already present).
In the ReadyNAS backup dialog settings, I used::
Name: Backup via Rsync over remote SSH (drop down selection)
Host: mydomain.com (or server IP address)
(Noting that if you followed the instructions for setting up ReadyNAS for SSH access correctly, no SSH passphrase will be associated with its public/private SSH key pair. This is rather important, as the ReadyNAS backup dialog does not provide for a passphrase)
At first this process did not work fully for me - using the ‘Test’ button in the ReadyNAS backup dialog with these settings did result in successful connection, but then initiating a backup generated error ‘Disk quota exceeded (122)’
But then a couple of hours later, with no further obvious intervention on my part, when the backup process initiated itself again on its auto designated hour, this time it just worked as it should.
Possibly my previous efforts at achieving a connection and testing using different settings were having some residual effect that impeded things, or perhaps being simultaneously logged in to the server from my PC as root via PuTTY at the same time as root via FileZilla (both using default key for root) and then trying to log in as root via ReadyNAS device backup utility (using ReadyNAS’s public key that had been added to root) was problematic.