The Lightning connector in iPhones only supports a single mono channel for mic level analog input. If you want a stereo line level input, you have to find something that provides a digital USB audio interface, and connect that to the iPhone with a Lightning to USB adapter.
There are a couple reviews that indicate this works properly with iPhones:
https://amzn.to/2Ug15YG
But even something as simple as this should work:
https://amzn.to/2I0tETD
But there's the question of how good the A/D converters are in those devices. I dunno.
Then you just need a Lightning to USB adapter. Either the official Apple one, or similar:
https://www.apple.com/shop/product/M...camera-adapter
https://amzn.to/2OupFjg