"Restrict background data" keeps turning off after a while

I have Android 6.0.1, original Samsung version on my Galaxy S5 Neo. Since I'm not on any mobile data plan and have to pay every MB I use, I'd like to restrict background data - once and forever! However, it keeps turning itself off after a while. (I don't know after what time - I often only notice it a few hours later.) So quite often, I don't notice that it's disabled again, then go online, and tons of stuff are downloaded, every single bit of which I have to pay... What might be the cause of this? (It's not rebooting - it happens without any reboots in between.) How can I make sure background data is simply ***always*** disabled? *(I know there are some firewall apps that might help, but they either need the phone to be rooted, or they route your data through some VPN. I don't like either solution.)*