Guitar input in 2020 MacBook Air and Garageband

I have been using GarageBand for about a year now, and I've been pretty happy and impressed.

I use it on my iPhone (13) and iPad Pro (5th gen), and have no problems at all.

So, I bought a MacBook Air (2020) and now I can't get my guitar to play into GarageBand.

It will happily record my vocals and all my MIDI instruments, but if I plug in a guitar I just get distorted noise.

Sometimes I get nothing at all.

I use a steinberg UR22C interface with my mic and guitar plugged into that, and feed the output into all devices via the USB-C socket.

I also bought an I-Rig HD2, but that gave me the same result.

So - any help would be gratefully received.