If I want to use cellular and Not WiFi for backup how can I do it?
Is it possible to change the settings of my iPhone from WiFi internet’s connection to cellular connection during I cloud backup??
iPhone 6 Plus, iOS 11.2.6
You can use cellular to backup but WiFi is recommended as the best option.
Stay connected to your Wi-Fi network until the process completes.
From -- How to back up your iPhone, iPad, and iPod touch - Apple Support.