Q: Accidentally deleted garageband app, how do I recover it?

Do you have a backup on your Mac?

via iTunes

See your purchase history in the iTunes Store on a Mac or PC - Apple Support

You never have to pay again for apps.

Log into the App Store using the Apple ID that you used to originally purchase or download the apps. Select the Purchases tab and download from there.

Assuming you are using the same Apple ID, go to the App Store and under purchased see if Garage Band is there with a cloud symbol.

Check the paragraph that starts with "If you purchased a device with iOS 9 or later that's 16GB or smaller, or ..." here:

Get Pages, Numbers, Keynote, iMovie, GarageBand, and iTunes U for free on your iPhone, iPad, and iPod touch - Apple Supp…

I would contact iTunes support since your phone seems to qualify.