I've tried everything but nothing seemed to work. Lastly I thought I'll backup my old iphone with assistive touch settings turned on and I restored the backup to the new iPhone and luckily assistive touch appeared on the screen. Not sure if it works for you, but give it a try. Update (10/16/2018): There’s few options for it. When I had the same issue with my iphone 7 plus, I made a backup with assistive touch on a different iphone and restored it. If it doesn’t work for some reason, just shut down your device and start it by holding volume down+power button, then it will by pass the home button screen.