Use a Bluetooth MIDI device with GarageBand for iPhone
You can connect a compatible Bluetooth® MIDI device to your iPhone, and use it to play and control Touch Instruments in GarageBand.
Tap the Settings button in the control bar.
On iPhone SE or iPhone 8, tap the Settings button in the control bar, then tap Song Settings.
Tap Advanced, then tap Bluetooth MIDI Devices.
Tap the name of the device you want to connect, then tap the Connect switch on.
To stop using the device, tap the Connect switch off.