I want to use Google Photos to back up my photos from my iPhone SE running iOS 10. I'm connected to Wi-Fi. I start the backup process and I connect my phone to power and then I lock it. It should just sit there and happily back up my photos. But if I come back to it after a few minutes, it seems like it had disconnected from Wi-Fi and that it only just reconnected after I unlocked it again. When I open the Google Photos app, it has to start backing up all over again.
How can I keep my iPhone connected to Wi-Fi while it's locked? (Like it should be!)