USB KEYBOARD:
Connect the keyboard to your mobile using the proper cable/adapter (USB-C or other).
BLUETOOTH KEYBOARD:
Activate Bluetooth on your keyboard, then go to iShala app > Settings, and there click on the BLUETOOTH MIDI button (You may need to grant permission for that)
You should then see you keyboard in the list there.
Click on it and wait till it shows the status Connected.
You then need to click on the MIDI Devices button and select your keyboard there.
After that, you can select the destination through the button below. By default it's set to Harmonium (Melody), but you can play any of the other instruments if you wish.
Finally if you want to use the modulation wheel of your MIDI keyboard to fluctuate volume, click on the Expression button and select the CC (Controller Change) number your keyboard is set to (by default 1 or 11).
You should then be set to play the voice of your choice through this external keyboard.