Sidebar with widget not showing

Hi all,

I have this front-page.php and it ends with an area for widgets. I do this using the following code:

<section wp-register-sidebar="Frontpage Widget Area" wp-register-sidebar-id="frontpageWidgets" wp-get-sidebar="frontpageWidgets" wp-get-sidebar-set="content" wp-dynamic-sidebar="frontpageWidgets">
widget area

Although I can add a widget using Appearance - Widgets, I cannot add Widgets trough the customizer. So I am forgetting something but I have no clue what.

All hints welcome,

Thanks and stay safe.