How can i use cellular data to backup WhatsApp on WhatsApp with an iPhone 7

WhatsApp is not an Apple product. Contact the app developer here --> App Support or WhatsApp FAQ  Or Restoring your chat history - WhatsApp FAQ or Contact WhatsApp

You can not backup your device on iCloud using your Cellular data. Note: iCloud device Backup does not include WhatsApp Backup which is done separately on iCloud using "Chat Backup" from within the app (WhatsApp). To know how that works...

WhatsApp is not an Apple product. Contact the app developer here --> App Support or WhatsApp FAQ  Or Restoring your chat history - WhatsApp FAQ or Contact WhatsApp