Here are the few frequently asked question.
If you have previously downloaded Instagram using your Apple ID, you can download the last compatible version directly to your device. on your iOS 9.3.5 device.
If the App Store method fails, you can manually install the application package (IPA).
To side-load an IPA without a paid developer account, users utilize a "7-day signing certificate." Apps signed this way expire after 7 days, requiring the user to connect to a computer and re-sign the app weekly. Furthermore, the login token may expire, locking the user out of their session.