I use an external Aux switch to toggle the tuner easily as shown in this video: https://www.eventideaudio.com/community/videos/h9-quick-tips-tuner

With regards to using CC to toggle the tuner, did you assign a MIDI CC to the "toggle tuner" option under the MIDI CC Receive Map using H9 Control? You find it under the "Pedal" page > MIDI Settings > Assign MIDI CC Messages > Toggle Tuner. Assign it a controler number. Once that is set, you need to program your MIDI controller to send the CC change on the MIDI channel of the H9 in question whenever you press the controller's footswitch. Some MIDI controllers allow you to connect an external switch to it for this purpose, so that you're not wasting a preset button for engaging the tuner.