How can I backup my iPhone without wifi

It isn't possible to backup it iCloud without wi-fi if you are asking about the iPhone 6 Plus. It is only possible to backup to iCloud without wi-fi in limited circumstances. See the following from Back up iPhone - Apple Support

Note: On models that support 5G, your carrier may give you the option to back up iPhone using your cellular network. Go to Settings > [your name] > iCloud > iCloud Backup, then turn on or off Backup Over Cellular.

You can backup to a computer without using wi-fi. See: How to back up your iPhone, iPad, and iPod touch - Apple Support