Actually, your MIDI assignment is correct here. Under “Control Source” where it says Off that is referring to Aux switches and Exp pedals.

I believe this is an issue with your MIDI controller setup, can you confirm it is actually sending CC# 0 with a value of 64 or higher? You can use a MIDI monitor to confirm this.

I can turn the tuner on/off with CC #0 so it seems that the issue is somewhere in your setup. You can also turn on “Receive Omni” on the H90 and that will make it easier to troubleshoot if your controller is not using the correct MIDI channel.