When the djay2/vjay master out is set to the controller, the volume doesn’t change even when I operate the Master Volume control on the djay2/vjay’s GUI. [djay for iOS]

This is due to the specifications of djay2/vjay. Adjust the volume on the DDJ-WeGO2 itself. Set the volume in the djay2/vjay application to the maximum setting, then use the MASTER VOLUME control on the DDJ-WeGO2 to adjust the volume.