There’s an iOS restriction called “cellular data changes”. If it’s enabled a user cannot change their cellular data settings.
I think the iOS 12 update defeats this lock. Settings will show “Cellular Data Changes” - “Don’t Allow” but the settings can be edited.
To reenable the restriction turn it off then turn it on again.
I’ve seen this on a couple of phones post iOS 12 update. I think it’s an old bug.