1. First, JAILBREAK using
3utools.
2. Install
AppAdmin from Cydia, first! (must add beta.unlimapps.com as repo sources).
3. Go to AppStore, search for WhatsApp. When you press DOWNLOAD, you will se a DOWNGRADE option. Select version 2.18.80 and install it.
4. Set the phone date to December 2018. Enter WhatsApp 2.18.80 and edit your phone number, DO NOT PRESS "DONE". Leave WhatsApp open, go to SETTINGS and put the correct date and time.
5. Go back to the opened WhatsApp (switch app using the Home button) and press DONE, to confirm your number.
You have successfully registered WhatsApp 2.18.80 to your phone.
You may now proceed to
UN-JAILBREAK your phone..... Restore to a clean 7.1.2 iOS, download WhatsApp from AppStore.
...Put your phone number, you are all good, working WhatsApp and no jailbreak.
If it helped, press THANKS.