Best App to Record Voice over a Backing Track?

Hi!

I'm a singer, though not professional, and like to sing covers with pre-recorded backing tracks, either played by me, downloaded/bought or played by a friend. I was looking for an app that I could use which would let me record my voice over it, with the audio I could then export.

I've attempted to use GarageBand and it worked for a time but recently it's become less accessible for what I wish to do. I have an iPhone 5s running iOS 10.3.2 and I also have a Macbook Air.

If anyone could help me, that'd be great!