Using Monoleap with GarageBand iOS
GarageBand for iOS, and several other DAWs, such as multitrackstudio for iOS, do not listen to MIDI coming from another app running in the background, but using AudioBus, it's possible to create a MIDI chain to overcome this issue. Simply open AudioBus in the background, and create a simple MIDI chain, selecting "Virtual MIDI" for both input and output.
Then keep GarageBand running in the background (making sure background audio is enabled) and switch to Monoleap. Now you should be able to play GarageBand instruments using Monoleap.