I am using Macos 10.15.7, and Garageband 10.3.5. I have a MIDI keyboard plugged in via a USB cable. I configured the MIDI device using "Audio Midi Studio.app". I added my keyboard, and tested it using Midi Studio -> Test Setup. The test was successful, the Midi Studio App plays sounds on my external keyboard.
When I run Garage Band, under Preferences -> Audio/MIDI -> Devices my keyboard does not appear in the list of available devices.
I've read articles on how to debug the USB connection to the keyboard, but from what I can tell (including a successful test where I hear the keyboard playing) that is fine. It's just that Garage Band doesn't see the device.