It's my understanding that you would need to be using a 5G enabled iPhone model for the option to Backup using Cellular Data to appear.
So perhaps, your iPhone, like mine, will
only backup over WiFi. I find this happens when connected to WiFi on my charger o/night.
If/when you have a 5G capable iPhone you will find a menu in Settings called "Data Mode", this is explained more fully in this link.
Use 5G with your iPhone
I only have an iPhone SE 2 so although I'm running iOS 16.6 4G is as high as I can go and the menu described in the link above does not appear.
Bottom line is you don't have to turn the option off because the default is WiFi unless 5G is available to you and even then you have to turn it on.