Bluetooth Midi Keyboard turns your iOS and iPadOS devices in a wireless keyboard. 
Play your iOS, iPadOS, desktop or hardware BLE Midi DAWs and instruments from your iPhone and iPad.
-Selectable keys number.
-Selectable octave.
-Bluetooth Low Energy Midi.

How does it works:
Push connect button and in the opened view turn on the switch button. This will make your device discoverable by your Bluetooth MIDI Host, connect it from it and you will can play your MIDI Host from B.M.K..
If your MIDI Host don't support Bluetooth MIDI there are some free apps on the market that creates virtual Bluetooth MIDI Ports.
B.M.K.. uses MIDI channel 2.

Demo video on the developer's page.