"Download an older version of this app? The current version requires iOS 11.0 or later, but you can download the last compatible version" The Result
You will not get a working Instagram experience, and you risk malware, account theft, or a permanently damaged device. The safest and most practical approach is to use Instagram through Safari on that device or accept that iOS 9.3.5 is obsolete for native app usage. Instagram Ipa Ios 9.3 5 Download
Use a service like Signulous or AppDB Pro ($20/year) to sign the app for 365 days. "Download an older version of this app