Studio 2.2 Constraints Layout with tab bar fragment

I’m fairly new to Android coming from iOS but have just taken the plunge with AS 2.2 and want to get to grips with the new constraints layout. I’m ok using it on an activity but have just set up a small project using the tab bar template with a fragment for each tab.

When I converted them to the constraints layout if I constrain a textView to the bottom constraint it shows on the screen as expected, i.e. just above the navigation bar (is it called that right at the bottom with back button etc?) but when I run it its like its constrained right to the bottom of the screen and therefore behind the nav bar.

I’ve been trying various possible solutions from the web but being new to android i’m not sure they would address my scenario.

This is preventing me going any further on this app conversion from iOS

Any pointers very welcome

Many Thanks