I’ve done other tests, but I forgot to restart with power and home (I restarted with normal power off/power on)
I uninstalled Audiobus 2, retest without success, then I installed Audiobus 3 and retest without success.
But if I use Audiobus3, I can route channels 3&4 to MusicStudio and it works (I will use this setup).
Another app, TE Tuner has the same issue (Channel 1,2 works, Channel 3,4 doesn’t even if I can choose them in the App Preferences).
So maybe it’s an IOS 11 issue.
Other apps like JamUp or AmpKit doesn’t allow to select channels 3&4.
I will retest with power on/off with home/power buttons.
Have you Steinberg UR interfaces to do some other tests ?