Well... I have managed to activate ssh and remote login to mine ... but that alone does not mean one can install a 3rd party firmware.
me@myhost test % ssh firstname.lastname@example.org
Last login: Sat Nov 14 14:43:22 2020 from 10.0.0.66
Terminal type is xterm-256color. ...
Right having finally freed up the rackstation from the backup server, I upgraded the Rackstation OS and it rebooted itself... I can now see the RX1217 in storage manager.
So to my original question for Synology support... do I need to reboot this for it to see the disks? Err YES!
Good old turn it off turn it on saves the day !