Roadster I got it working and this is what I found. First - I tried different lightning to USB adaptors and different USB A to B cables but that wasn't the problem. Not saying that couldn't be a potential issue but for me it wasn't.
So the app needs to be on and the iPad hooked up to the mixer before turning on the mixer. If this doesn't happen then you will see the mixer in the app but no sound gets sent. Weird but that is how it works.
Next, make sure your routing is correct, I made the mistake thinking the soundcraft ui24 numbers I see in the app correspond to the mixer channels but they don't. What you see in the app are the usb slots. You then need to go to the mixer and patch those slots to a channel. I ended up using slots 1-8 in the app and assigning them to channels 17-24 in the mixer.
So far so good it has been running stable, connecting and working as expected over the last few days.