You need a factory reset. Back up your data via OPPO Clone Phone or Google Drive first. Then go to Recovery Mode → Wipe Data/Factory Reset .
Major system upgrades often come with temporary "settling" bugs. If you experience issues after moving to 7.1.1, try these fixes: : oppo f3 nougat 711 official update download fix
We are not responsible for any damage or issues that may arise during the update installation process. It is always recommended to backup your data before attempting to install any software updates. You need a factory reset