Device Tracker/Extended Tracking is not working. What am I doing wrong?

Hi

I have installed and set-up the latest Vuforia with Unity (v2018.3.7, so Vuforia came installed with Unity).
This video shows you that it works: https://imgur.com/bFYERdp

But it also shows that Device Tracker/Extended Tracking is not working. The model disappears when the Image Target (the book) is out of sight. And I eve carried out the correct step (https://imgur.com/9EWHR6K)

What am I doing wrong? I can't think of anything as its only 1 step for enabling this...

Device Tracker/Extended Tracking is not working. What am I doing wrong?