Show full screen baner/header on dynamic topics page

WP 5.4
TwentyTwenty base Theme

Hi everybody,
Certainly miss something but I can’t exploit any cover-header with static homepage for topics set up.

On any other page, I can choose à cover picture and use it as “Template with banner” in fullscreen.
But this function is displayed off when I set the asked page as official “topics page” in Customize > Homepage Setup > Topics Page.

The website is right now on my local WAMP server so I’m sorry to can’t share with you

Could you please guide me on any good track ?

Thank you all and keep a eyes on your health while this period, buddies.