Do you have DN set to Overbridge? Switch to USB-MIDI.
I was about to try it out but can’t find my Lightning to USB adapter.
Update - It’s now working for me.
I have the DN connected to my Lightning to USB adapter and in to the iPad. Selected a Midi track on the DN and set channel to 1. Make sure DN is set to USB-MIDI and NOT Overbridge in settings.
In AUM click on Midi-Routing - you should see the DN port there and an orange Midi indicator line flash when DN keys are pressed.
In AUM main screen add an Audio track (not MIDI) and set an instrument (I am using Arturia iSEM. Go back to the AUM MIDI-routing page and link DN MIDI out to iSEM in.
@Zenji - does this work for you now?