That is a common problem after an OS update.
My solution to DATA plan protection is the same as it has been ever since the first cellphone was released to market.
Always keep DATA turned OFF. It is not needed for 95% of the things we do thru the day. And when I am home, it damn sure is not needed at all because the WiFi router is always ON.
The few rare moments, when I am driving in a metro area is that I need to find an address, or use Google Maps to give me 'Turn by Turn" instructions.... then I turn DATA ON so the Internet is accessible.
I also use a Macro to turn WiFi OFF when the screen is locked.
This saves battery power because it stops all apps from trying to "call home to mama" and download useless trivia junk that I never use anyway.... Sync behind the scenes is the most useless piece of crap on our phones. If I want to know something is updated, I will open up that specific app and go look to see what is new.....
Why should I waste battery power on the other 115+ apps loaded on my phone?
I only use the browser, email, news apps, FB, and my motorcycle forum app.
All of the other tools we have downloaded? For me, they are just that, Tools to be used "as needed"....
I do NOT need to know that the Weather Channel has an important alert for Tennessee when I live in Oklahoma... so why should I allow the Weather Channel to use up battery needlessly?
bottom line, keep the Internet turned OFF unless you need to use the phone.
2nd, go to the Play Store and My Apps, and turn OFF "Automatic Updates".
that is a useless piece of nonsense IMO, rarely have I ever needed to update an app... none of them have ever broken, and they have all performed just like they did when downloaded and I liked it, and kept it.
I don't need to have that same app updated because some 13 year old kid learning how to program decided to change just one line of code in the app, and now he thinks that I need to "Update" the app on my phone. Sorry snot nose brat, I don't.... the app works just fine the way it was the first time I got it, and kept it.
The apps that don't perform to my standards are deleted immediately.
Long long answer to your problem, but it makes a Point!
a useless OS update, which in turn Requires that all the apps on your phone get updated also, uses up your DATA plan minutes/megabytes.
None of that can happen if you practice what I preach.
Indeed, my wife's S5 is still on 4.4.4 and is unrooted and on Verizon. The worst of the lot of carriers to force their crapware onto us.
how did I keep VZW from forcing her phone to upgrade to Nougat?
Simple, I use a macro tool, MacroDroid, to always turn DATA and WiFi off every time the phone wakes up, and when it goes to sleep. Her phone has never seen the Internet. for her, it is a telephone, period.