Try to perform a restore from recovery mode. https://support.apple.com/en-us/HT201263, this will put a fresh copy of the iOS on you device. Note make a backup of your iPhone first. When you set up it choose "set up as new and test."
I work for said mfgr. Put your device into recovery mode and restore. Note: you will need to watch iTunes for recovery mode because you cannot see it on the screen