When you say "I get nothing", I assume you mean that literally nothing is working.  The Sunday Keys app doesn't detect any MIDI from your Motif and the audio output doesn't sound.

I'd start by simplifying if you haven't already.  What I mean by that is to remove the hub and the interface (Presonus AB96) from the configuration.   Connect the iPad straight to the XF8 straight with a USB cable.  Try to get Sunday Keys to recognize MIDI from the Motif in this reduced complexity configuration.  Make sure the MIDI channels match up.  Maybe take some other app on the iPad that will show (monitor) your MIDI traffic (MIDI Wrench, etc).  Just to make sure something is happening when you mash down on keys.  Check the MIDI channels the monitor reports data is coming in as.  See if Sunday Keys lets you configure this as an input channel config.

I'm assuming from your audio interface choice, you've already seen the video manual: https://www.youtube.com/watch?v=wrURNDxq6x8

It mentions that the app should automatically detect your keyboard.  So perhaps you're plugging your USB cable into the wrong port on the Motif XF8.  It needs to be connected to USB TO HOST which is a "printer cable" style connector ("USB-B" ).  Note that in the video that another recommended connection to your keyboard would to use a USB-to-MIDI (5 pin DIN) cable.  It's unlikely you have this cable but it would be perhaps slightly more prone to working out of the box.  The video shows a specific product they implicitly recommend.

Once this is working you should see the onscreen keyboard start to move from pressing keys on the Motif.   You may also have to configure the Motif XF.  

All power OFF - power up Motif FIRST - then lpad and other devices.

Sometimes a device won't detect the instrument if it isn't already on. Yes - I know it is supposed to - but it is worth trying.