Using iPad as an external synth

Hi I have a MAC MID 2010 with OS Sierra and an iPad (the first iPad that came with lightning cable). How do I set the iPad up so it can be used as an external synth?

My sound card in the MAC is a digital RAYDAT and I have an external MIDI keyboard. I also have a mixer. I need to both record the MIDI signals in Cubase but get the sound from the iPad and then render or something to audio.

What kind of cables do I need? Which menus must I access to set it up?

Buy a MIDI to USB device, that is supported by the Ipda, then buy a device that converts the analog ipad audio outputs to ADAT format, then read the chapter „external instruments“ in the manual.
Or get a USB audio / MIDI interface that is supported by the ipad. And again check the „external instruments“ chapter from the manual.