@lijas90 In your callback you need to do 2 things.
Hide/show the panel
Set the attribute of the LFO.
You're already doing the first one, so just one step to go. I have a ton of videos where I show how to set module attributes from controls - it's the same for all component (knobs, buttons, combo boxes, etc.). The only difference is that with a combobox the first value is 1, not 0.