To get WhatsApp running, you need an (iOS App Store Package) from a version that existed between 2015-2017. What is an IPA File?
Understanding the Challenges: iOS 7.1.2 and WhatsApp Server Architecture
Input your Apple ID credentials (using an app-specific password is highly recommended for security).
He tapped it. The chat history was gone—wiped from the server side years ago—but the contact info remained. He typed a message: I finally got the old phone working. Miss you.
The short answer is that WhatsApp no longer works on iOS 7.1.2
You must locate a legacy WhatsApp IPA (typically version 2.18.x or older) archived online via reputable repositories like the Internet Archive or specialized legacy iOS forums.