Hi @nicolai 
welcome to backstage!
So they’re connected via USB only, not using midi cables right?
I’m not sure and I couldn’t find the threads I remember from a quick search but if I remember correctly the tracker doesn’t act as the midi “host” via USB. I have the mini but I think it’s the same limitation for both.
If you have AUM you could try with the 3 connected to route the midi in from the keyboard out to the tracker and that should work. If you don’t want to use the iPad there are some devices that can act as midi host, I think https://retrokits.com/shop/rk006/. Or connect them to the hub for power and use an additional midi cable from the keyboard to the tracker.
Actually searching for the rc006 turns up some posts on the topic: