decreasing data usage

I had called into Koodo at one point and they  navigated me to a toggle button that when activated decreased my data usage.  I believe the effect of it was that my phone would look for wifi first - I can’t really remember.  Anyway - I would like to be able to find that again to use it on the other phones in my plan but I can’t find it again.  Would love to call but a callback tomorrow isn’t helpful.  :(   Let’s see if this 15 mins is really a thing….