Problem with MB Settings Page value

Hello, I have a problem with the Meta Box Settings Page plugin, I create all the field and there are display on the admin page but I try to display it on functions.php or in header.php, and it doesn't work... I use OceanWP theme, I don't know if is the problem ?

/* META BOX SETTINGS */

// Register settings page. In this case, it's a theme options page
add_filter( 'mb_settings_pages', 'ac_options_page' );
function ac_options_page( $settings_pages ) {
    $settings_pages[] = array(
        'id'          => 'ac-setting',
        'option_name' => 'Avantage Courtage',
        'menu_title'  => 'Avantage Courtage',
        'icon_url'    => 'dashicons-admin-settings',
        'parent'      => 'themes.php',
        'style'       => 'boxes',
        'columns'     => 1,
        'tabs'        => array(
            'taux' => 'Taux',
        ),
    );
    return $settings_pages;
}

// Register meta boxes and fields for settings page
add_filter( 'rwmb_meta_boxes', 'prefix_options_meta_boxes' );
function prefix_options_meta_boxes( $meta_boxes ) {
    $meta_boxes[] = array(
        'id'             => 'ac-tx',
        'title'          => 'Taux du Calculateur',
        'settings_pages' => 'ac-setting',
        'tab'            => 'taux',

        'fields' => array(
            array(
                'name' => 'Taux sur 15 ans',
                'id'   => 'ac_tx_15',
                'type' => 'text',
            ),
            array(
                'name' => 'Taux sur 20 ans',
                'id'   => 'ac_tx_20',
                'type' => 'text',
            ),
            array(
                'name' => 'Taux sur 25 ans',
                'id'   => 'ac_tx_25',
                'type' => 'text',
            ),
        ),
    );
    return $meta_boxes;
}

function get_theme_option($field_id)
{
    global $settings;

    $value = (isset($settings[ $field_id ])) ? $settings[ $field_id ] : false;

    return $value;
}

         $settings = get_option('ac-setting');
            var_dump($settings);
            var_dump(get_theme_option('ac_tx_15'));
            var_dump(rwmb_meta('ac_tx_20', array( 'object_type' => 'setting' ), 'ac-setting' )); 

Can you help me ?

Thanks