Connect your device via USB and use the sideloading tool to sign the app with your Apple ID. Important Limitations
that still support iOS 9:
Even if you manage to install an old .ipa file, the WhatsApp servers will block the connection, as older versions are phased out to maintain security and encryption standards . Alternative Methods (Limited Success)
for an iOS 9-compatible version (like version 2.18.x), the app will prompt for a mandatory update upon opening. Since your hardware cannot run the new update, the app remains locked. WhatsApp Help Center 2. Why Downloadable .ipa Files Fail