When you open WhatsApp and enter your phone number, the app connects to WhatsApp’s modern servers. Because your version is outdated, the server will block the connection and display a critical error:

Drag and drop the downloaded WhatsApp .ipa file into the Sideloadly window.

How to Download WhatsApp Old Versions for iOS 10.3.4 (Step-by-Step Guide)

Once the app appears on your home screen, go to your iPhone's . Tap your Apple ID email and select Trust . Crucial Caveat: The "Expired Version" Server Block