Nicola is right. IF you want to have a musician on stage control their aux monitor mix the GLD OneMix is the right application to use. It’s a bit unfortunate it ONLY runs on iOS or OSX but as you can live with that constraint, each device can be assigned to a specific aux.
Keep in mind that this has to be done by signing into the One Mix app using the ADMIN profile, setting up the device for the Aux you wish your user to control, setting up the layers with the instruments they would like to control and then signing out again.
Once done, the musician can sign in using their non admin account and start setting levels.
Be advised that there is almost no help on how to use the app. (poo poo).
Only audio engineers and trusted volunteer should use the GLD Remote app. Things can go very wrong very fast otherwise.