Hi, just from my general experience, midi controllers used with GarageBand on iOS can only use the basic features like keys, mod wheel, and pitch bend wheel. There isn’t a way that I know of to assign midi to the knobs, sliders, etc…
On a related topic, I picked up the Minilab 3 recently for use with GarageBand on the Mac, and was testing it out with GarageBand on iOS and can not get any sound when I play the keyboard. I am using a direct USB C to USB C apple-branded cable.
Perry