Originally Posted by
mickeyjaw
If you use Android instead there is an app called "Roaming control" which can do exactly what you are asking for. I have no idea if there is an equivalent for iOS.
Handy app but won't work for anything running Android 5 or greater because of security changes. There are a couple of others out there which can alert but AFAIK none now can toggle roaming off due to security policies. Roaming Guard is one.