Thanks. I see there is a toggle button for this in the pull down menu. But remembering this each time I leave the house … 
and especially not undoing this when I enter could waste my mobile budget. But if it works, it might give more insight in what is going on.
What also frustrates me is that I don’t feel this is an issue with my phone specifically but with either FP or e/OS. Yet there is almost no one having a similar problem. Only: Phone doesn't connect to mobile data after leaving home wifi