offline turn by turn?

Hi, new traveller app user here.

I'm running the app and premium maps on a Samsung Galaxy Tab A tablet.
I have not been able to get offline turn by turn navigation working (using private saved trek routes or existing Exploroz treks downloaded and accessed via 'go to' command). As soon as i go offline the navigation box where the options for drive/walk/cycle usually are just has a message saying 'unavailable' and it reverts to 'fly' direct connection between points.

Anyone else had this problem? help appreciated

Brad