How do I restore my iPhone when I can't turn off find my iPhone

My phone 6+ has completely frozen out. It will turn on but there is a flicker at the top of the screen and no screen touching works. So I can't get past the security screen. No water damage, no magnet, not even dropped. It just stopped working one day.

I've tried to restore factory settings via iTunes, but it says that I have to turn off 'find my iPhone' first, which involves turning on the phone to do so, which the phone won't let me do!

How do I restore my factory settings?