How to stop app updates over cellular data?

I have a limited data plan and so have cellular data turn off for most apps, including the app store. But I do have automatic app updates turned on. Recently, after upgrading to iOS 11.1.1, I noticed that several app updates were needed. While in wifi, I initiated many of these manually, but they seemed to load very slowly (perhaps due to some internet issues) and several were still trying to update a day or two later. Then we had a power outage, the wifi signal was off, and suddenly my iPhone 7 has updated all its apps. My service provider shows a sudden spike in cellular data on this day -- running up my bill. Looking under cellular data, the usage amounts indicated over the past several months do not add up to half of the usage my cellular provider reports.

From this I suspect that my phone used cellular data to update my apps, even though cellular data is turned off for the app store.

How can I prevent this from happening in the future?