A couple years ago, I downloaded a file locker app that I used to store important information on my phone. The app has since been removed from the App Store, and being my stupid self I kept procrastinating finding a new app to use despite the warnings that this app would not work with iOS 11. My phone updated to iOS 11 and as I should have expected, the app no longer works and I cannot open it and access my data at all. I attempted to revert my iPhone back to iOS 10.3.3, and used a backup that I have saved on my computer that I know contains most of the information I lost, but when my iPhone attempted to redownload all of the apps from the backup, the app I need disappears. Does anyone have any suggestions either on how to access this app in iOS 11, or keep the app from deleting when I try to revert my phone to the backup?