Turn Off Auto Brightness in iOS 11
Apple removed the Auto Brightness toggle from the Display settings, but you can still turn it off on iOS 11. You just need to know where to look. The video below shows you where the settings are, and you can read more below that.
Instead of looking in the Display settings, you need to go to Settings -> General -> Accessibility ->Display Accommodations -> Auto Brightness -> Off. And then you will be in full control of your brightness.