A little more on using Garage band on iOS, unlike a PC or MAC when your Audio interface is connected and selected as default it remains so, on iOS the default audio interface is always the built in speaker/ear phone jack for playback and the built in mic for input (or Bluetooth device if connected). There isn’t a global setting to set this and for the vast majority of people this works perfectly.
Apple assumes that if your intention is use alternate input and output then this is intentional and a special case, whereas the normal will be to always use the built-in offerings, this makes sense and so does the workaround as it avoids having to dig into and remember settings.
It’s as simple as opening the APP you intend to use, then, after it is open, connect the audio interface of your choice and the APP automatically defaults to the newly connected device for both input and output.
There are a few provisos here
1. That your audio interface is standards compliant, here, simple is your friend, I’ve used both a Zoom H2n and a Focusrite Solo, both of which worked seamlessly, but, for example my recently purchased Audient Sono does not, hardly surprising given the complex routing options, onboard tube, re-am switching and built in two notes wall of sound support. By far the best thing here is to keep it simple, definitely check with the manual and or manufacturer to confirm that your device is compliant, especially if you intend to purchase one to meet the requirement.
2. I recommend using the Apple camera kit to connect your device. While there are MANY thunderbolt USB dongles on the market with extra inputs for say simultaneous charging and headset connections, they often don’t play nicely with audio interfaces. Given Apple’s penchant for proprietary, they are constantly changing the goalposts for compatibility to basically ensure you buy their expensive branded accessories. In this case, while annoying and expensive, it’s better to play it safe here as ultimately the user experience, quality of recording and peace of mind will be better, as to the genius that thought up chipping accessories and software blocking to ensure market monopoly and bloated pricing, there is a special place in hell for you!
Once connected it’s pretty much plain sailing, you should be able to select inputs from the available ones on your device and that’s it really. There are plenty of iOS garage band tutorials out there, but it really is easy, so I would recommend putting on your big boy pants and getting on with it!
Troubleshooting
Most issues can be solved by disconnecting/reconnecting the audio interface (I recommend disconnecting the dongle itself as that’s how iOS recognizes something has changed, it’s possible if you only disconnect/reconnect the USB cable that iOS will not register it) or closing the app completely from the iOS multi-tasking screen (push the home button twice, find the app, then swipe up to close it). Then disconnect the USB dongle. After you have re-started the app then reconnect the USB dongle.