MIDI FX Transpose Question

Hello all, new Mainstage user here.

I am trying to learn how I can use the MIDI FX Transpose effect and setup 2 dedicated controller buttons where if I press one button, the transpose goes up one semi tone and if I press the other button that is mapped to my piano, it goes down one semi tone.

I can transpose my music using this Midi FX manually by using my mouse to adjust the keys up and down but this isn't practical when you're playing the piano. I can NOT figure out how to connect it to my controller so it I can modulate my music on the fly and say, for an example, change keys 2-3 times during a song by simply pressing a assigned button.

I have figured out how to connect it to my controller and modulate the keys 1 semitone easily but when I press the button again, instead of it going up and additional semi tone it simply reverts back to the original key. I am looking for a sequential setup where I can go up and down in semi tone increments just like a traditional transpose button would normally work.

What am I doing wrong lol?