dont have auto brightness anymore on ios 9.3.4

Hello thunderstuck23,

Thanks for reaching out to the Apple Support Community. I know screen brightness makes a huge difference, especially if you like to conserve your battery like I do. I'd like to help out with getting your Auto-Brightness back in working order. When we see behavior such as this it's a good idea to start with the settings. If you go to Settings > Display & Brightness try toggling Auto-Brightness off and back on again.
If the option itself is missing from your settings, try restarting your iPhone by pressing and holding the Sleep/Wake button and then sliding to power down. When you restart your iPhone if the option for Auto-Brightness is still not working or not visible, then it may be a good idea to backup and restore your iPhone to ensure the latest update is installed properly.
This article will walk you through making a backup of all of your data: How to back up your iPhone, iPad, and iPod touch
This article will help you restore the iPhone to factory settings to re-install the operating system: Use iTunes on your Mac or PC to restore your iPhone, iPad, or iPod to factory settings
One you've restored the iPhone you can then restore your backup using the steps here : Restore your iPhone, iPad, or iPod touch from a backup
If you run into any trouble or have any questions the entire community is here to help.
-Best Regards-

Hello again thunderstuck23,
Since the issue returned after restoring, it may be a hangup in the actual settings of the phone, which would be restored with the backup. To correct that we would recommend resetting the general settings. This does not remove any data like Apps, Photos or contacts, but it removes your preferences for Wi-Fi, Bluetooth, Do Not Disturb, Notifications, General, Sounds, Touch ID, Brightness & Wallpaper and Privacy, so we'll have to set those things back up after this reset which may take a few minutes.
To do this reset you can go to Settings > General > Reset > Reset All Settings.
If you have any other questions, please reach out again, as always the whole community is here to help.
-Best Regards