This is the most reliable method but requires a PC or Mac.
Note: A free Apple ID only signs apps for 7 days. You will have to re-sign them weekly unless you pay for a developer account. Ipa Apps Ios 9.3.5
If you are willing to jailbreak, you gain access to Cydia, which allows you to install apps that will never expire and modify the system to run apps that usually require newer iOS. This is the most reliable method but requires a PC or Mac
"Legacy Software in a Modern Threat Landscape: A Security and Usability Analysis of IPA Applications on iOS 9.3.5" Note: A free Apple ID only signs apps for 7 days
3uTools is a third-party management tool that resurrects old iPAs.
This is the easiest method if you don't have a computer handy.
Fix: Your device is 32-bit (iPhone 4s) and the IPA is 64-bit only. Find an older version of the app (pre-2015).