Tacx Desktop app not displaying Heart rate

I recently purchased a Tacx Flux 2 trainer, which I  use in conjunction with : 

1.  Garmin Edge 520  

2.  the Tacx Desktop app.

3.  Macbook pro (using the latest version of macOS Catalina)

The Tacx app works well,  and is a pleasure to use with great films and displays all the relevant data with the exception of my Heart Rate.  

I have spent numerous hours trying a variety settings on the 3 components but cannot get a Heart Rate reading on the app. My Edge 520 continues to show my HR in the usual way.

I would welcome any advice on the settings that I should be using to get the Heart Rate display.