Is the LED on the disc red as well? And which firmware version are you using?
It might be worth manually restarting (switch off/on) all of the discs as well.
I would try factory resetting the single red one first and re-add it with the app. It seems like the system is confused and there are two "primary" or "master" discs.
just to close this off. I had to reset all the disks to factory settings and start again. Crazy to have to do this but only way to solve the issue.
Now trying to find a way to extend the network further as four disks don't cover everywhere.