I’ve got the same problem. I’m using the UR 44 with an iPad Air 2, Apple Camera Connection Kit, the latest version of Garage Band, and both iOS 10 and 11.
Garage Band sees all the inputs, and they can be selected, but there is no signal. I tried the exact same set up with an Alesis Core 1 interface, and it worked perfectly, so I can only conclude the UR44 no longer works with Garage Band.
The original box says that it’s compatible, but I can find no mention of Garage Band on the Steinberg site or on major retailers’ sites. Has Steinberg quietly (no pun intended) ditched iOS Garage Band support?