If you haven't already, you could try to update the firmware via a web browser rather than the app (which I've found can be temperamental):
Login using this web address:http://mybtdevice.home/index.htm
Input the admin password for the WH Discs and click on 'System' tab>Firmware Updates.
What device is the WH App on? I've read on here that when some have had problems using the app, deleting it from the device and re-downloading a fresh copy and starting over can get things working properly again.
I have the app on an iPhone and an iPad and have in the past had to re-install the app to get it to function correctly.
Just one last thought - in trying to add the new discs to the network have you first plugged them into you router via an ethernet cable to allow them to sync all the connection details before proceeding with the 'add new disc' process.
I wonder, if the new discs were left plugged into the router for several minutes, whether they would self-update the firmware on them to match the firmware of the existing discs, which sound like they are on the latest version v1.02.05 Build 10. Just guessing at ideas here...
If you plug the new disc into the router does the light eventually go 'blue' indicating that it has completed whatever it's doing, or does it stay red and never complete the process.
If it does go blue have you tried unplugging it and then starting the 'add new disc' process via the app. (I'm not quite sure at what point you are seeing the message that is stopping you proceeding with the 'add disc' process).
The only other thing I can think of doing would be to factory reset the new discs using the button on the back and start over, first connecting to the router and then following the instructions in the app.
Other than that, I'm afraid I've run out of ideas, hopefully someone else will spot this thread and have some other suggestions. Good luck