At first, open Settings and go to Mobile Data. Then scroll down on the same page. You can find all the apps that use your mobile data.
By default, almost all the buttons should be turned on. You just have to toggle the button turn them off. You can turn off any number of apps that is listed on this list. Following that, whenever you will try to open an app, you will get an error. Therefore, you will have to provide Wi-Fi connection to run that app on your iOS device. Otherwise, you cannot work with that app using mobile Data.
