If you have Two-Factor Authentication (2FA) enabled, the old app might not show a text box for the code. You may need to disable 2FA temporarily to log in. The Web Alternative
Even with a successful installation, using Instagram on a decade-old operating system comes with hurdles:
This guide explores how to find, download, and install a working Instagram IPA for iOS 9.3.5. Why You Need a Specific IPA for iOS 9.3.5
This community focuses specifically on keeping "whitelisted" legacy devices alive. Their forums and App Store alternatives often host verified versions of Instagram that still communicate with modern servers. How to Install the IPA on Your Device