How to use headphone jack as line-in for GarageBand (iPhone X)

I was given a Teenage Engineering "Pocket Operator" (which has an analog audio output) and I'm trying to record that audio onto my iPhone in Garageband.

I plugged a 3.5mm Male to Male stereo audio cable into the Pocket Operator, and connected it to my iPhone via the Lightning to 3.5 mm Headphone Jack Adapter (which was included with my iPhone)

However, Garageband doesn't seem to recognize the audio coming in. It's only recording the ambient noise from the room.

Do I need another adaptor? How do I plug in a device and record analog audio into Garageband???