I have this same/similar problem. Using Mohave. I always get the standard accessibility keyboard when I use the menu bar item to Show Keyboard Viewer This standard Accessibility keyboard has grayish keys and does not have a button for Panels.
I have not found the trick to make the menu bar item show the keyboard that shows up when I check off Enable Accessibility Keyboard. Whenever I dismiss this Accessibility keyboard (hitting the X) I have to navigate back to the Accessibility Keyboard area and recheck the check box for Enable Accessibility Keyboard which will have become unchecked.
I am experimenting with just displaying the keyboard on the Mac itself, My thought is that if I can get it to work on the Mac display that I can make the step to buy the Luna display, but I cannot get past this hurdle.