Just checked, it's available and got a new update 3 days ago.

I think you should use this plugin, put the widget before the shortcode in the forum page editor:

https://wordpress.org/plugins/amr-shortcode-any-widget/

You should contact this plugin developers and ask them to help you to only show this widget on mobile pages.

Or you can put this code into the functions.php of your current active WordPress theme. This will add new sidebar in Dashboard > Appearance > Widgets admin page, so the widgets in this sidebar will be only displayed on the forum top area on mobile devices:

function wpforo_custom_sidebar() {
    register_sidebar( array(
        'name'          => __( 'Forum Top Mobile Sidebar', 'textdomain' ),
        'id'            => 'mobile-sidebar',
        'description'   => __( 'Widgets in this area will be shown on forum top for mobile only.', 'wpforo' ),
        'before_widget' => '<li id="%1$s" class="widget %2$s">',
        'after_widget'  => '</li>',
        'before_title'  => '<h2 class="widget-title">',
        'after_title'   => '</h2>',
    ) );
}
add_action( 'widgets_init', 'wpforo_custom_sidebar' );


function wpforo_custom_sidebar_area() {
    if( function_exists('is_wpforo_page') && is_wpforo_page() ){
        if ( !is_active_sidebar( 'mobile-sidebar' ) || !wp_is_mobile() ) return;
        echo '<div class="wpforo-top-sidebar">';
        dynamic_sidebar( 'mobile-sidebar' );
        echo '</div>';
    }
}
add_action( 'wpforo_top_hook', 'wpforo_custom_sidebar_area' );

BTW, you can use Code Snippets plugin to add custom codes.

I would be good if i could just add one widget like "recent post" on top of the forum on mobile. So the members can see recent posts and not the whole side bar. It is a pain to scroll passed the whole forum to get to the widgets....

Any idea how to fix this?