The problem is that the scanner is pretending to be a Bluetooth keyboard, so the soft keyboard does not appear.
Ideas so far:
I have found this Show iPhone soft keyboard even though a hardware keyboard is connected, but according to Force software keyboard on iOS 10 this does no longer works as of iOS 8+
Apparently you can bring up the virtual keyboard by pressing the eject key. So if you can simulate that when the TextField gains focus, the keyboard should appear. Unfortunately CGEvent is only available for macOS
Is there a way to force the keyboard to show with SwiftUI?