If you only need a MIDI Connection, to control the synths and sounds in Garage Band, I believe the both cheapest and simplest solution is to get a Apple camera connection kit, lightning to USB adapter:
https://www.apple.com/it/shop/product/M ... ning-a-usb
This converts the Lightning connecter to a standard USB 2.0 port (the Lightning charging/sync cable that came with the iPhone won't work for this). Then you simply connect the Nord with the iPhone w Apple CCK, and a normal USB type A to type B cable, and you should have a 2-way MIDI connection between the Nord and the IPhone. Also be sure that the Stage 2 is set to a USB MIDI channel for the Global MIDI, external MIDI, and/or the other MIDI Parts you want to use.
If you OTOH want to record the audio out of the Nord Stage into GarageBad, you will need an iPhone-compatible audio interface.
I am using a Roland DUO Capture EX, which is very practical for iOS units, and it can run on three AA-batteries for portable recording, BUT it seems to have been discontinued. If you can find on, though, I highly recommend it. This interface still require the CCK Lightning adapter above, to worl with an iOS unit.
https://www.roland.com/global/products/duo-capture_ex/
The Nord audio outs of the Nord is then connected to the audio inputs of the Duo Capture EX, and if you still need MIDI connection with GaragBand, you also connect the Nord to the Duo Capture EX with standard Midi cables.
Then the most sophisticated solution, I think, would be to use an iConnectivity audio interfaces:
https://www.thomann.de/it/iconnectivity ... audio4.htm
It is one of the few audio interfaces that is made for iOS-units. It requires no Apple CCK adapter, and maybe more importantly - it also charges the iPhone when in use, if you use the supplied, special USB to lightning cable.
Then you connect the Nord to the iConnectAudio4+ with audio cables, and if you also want MIDI - standard MIDI cables, or a USB A-B cable between the Nord and the USB host port of the iConnectAudio4+
If you also connect a computer with the iCA4+, it can share both MIDI and Audio connection with the iPhone/iPad you use, so you can actually stream digital audio to and from the computer and iOS-unit - super cool!

I don't know of any other device (than ofther iConnectivty units), that can achive this.