Don't update Sony Xperia XA2

I have a Sony Xperia XA2. While away from home, I bought 1GB of mobile data. The phone was set not to do any updates while on mobile data.

Well, update came through, it kept displaying on the phone. And then it updated itself via mobile data, bypassing the settings. So the data was all spent. Buying more data, and changing the setting to wifi only, guess what, it tried again, but likely it was stopped in time.

So the phone FNB is selling to clients, bypasses the settings not to update, and uses valuable mobile data where it was not suppose to.

So taking the phone home, and letting it update on wifi, because it kept showing the update on the screen, so update done, now the phone is practically worse off. It is slow, takes 30 seconds to unlock, doesn't even open applications when you click on them.

Can't factory restore the phone, since it will force the update again. Can't give the phone back to FNB since I have to pay an early cancellation fee.

What should we do now?