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Hello,
I have find the solution in this site
https://sevenspark.com/docs/ubermenu-3/theme-integration/twenty-nineteen

You need to add a single function that will desactivate the main-menu-more.
Paste this code in a child theme functions file or as a plugin:

add_action( ‘wp’ , function(){
remove_filter( ‘wp_nav_menu’, ‘twentynineteen_add_ellipses_to_nav’, 10, 2 );
});

Hope it helps!