Hi Matthew,
Thanks for the quick reply. Here is attached the screenshot of the parameter set in balena cloud.
However, I finally managed to access to the parameter tab by pressing F1 on the keyboard !
Then, instead of mentioning the CONFIG_MODE parameter in your tutorial, I would simply recommand to press the F1 key.
Finally, I have another advice for other user regarding the jitsy adress : I recommand to add the code #config.startWithVideoMuted=false to the jitsy URL meeting to make sure that the webcam is set to ON when joining the meeting. Example of a jitsy meeting : https://meet.jit.si/test1234#config.startWithVideoMuted=false
Now everything work for me and I can’t wait to see the sparks in the eyes of my grandparents when they’ll be able to see us on their tv screen simply by pressing the switch on button of the pi !
Thanks again for your very good work guys and I’ll try to be helpfull for people who might have issues setting up the pi 
(I have a Pi 4B 4GB Ram with a fan case ordered on Amazon).
Best,
Paul