Mirror GarageBand for iPhone to an Apple TV
You can mirror the GarageBand app along with its audio output via Screen Mirroring to an Apple TV.
Mirror GarageBand to an Apple TV
Make sure your iPhone and the Apple TV are connected to the same Wi-Fi network.
Open Control Center, tap the Screen Mirroring button , then tap Apple TV.
The GarageBand window appears on both the iPhone and the Apple TV. Audio output is streamed to the Apple TV only.