Understanding How to Retrieve Taxonomy Advanced

I'm trying to retrieve the TaxonomyAdvanced from MB Settings Page, but I don't understand why I'm getting Null:

$meta_boxes[] = array(
		'id'             => 'activate_c',
		'title'          => __( 'Active c', 'textdomain' ),
		'settings_pages' => $prefix .'c_settings',
		'context' => 'advanced',
		'priority' => 'high',
		'autosave' => true,

		'fields'         => array(
			array(
				'name'    => esc_html__( 'Activete C', 'textdomain' ),
				'id'      => 'cc_activate',
				'type'     => 'checkbox',

			),
			array(
				'name'    => esc_html__( 'CC', 'textdomain' ),
				'id'      => $prefix . 'cc_date',
				'type'    => 'datetime',
				'timestamp' => true,
				'visible' => ['cc_activate', true] // Visible if checkbox_field is checked

			),
			array(
				'name'    => esc_html__( 'CC Select', 'textdomain' ),
				'id'      => $prefix .'cc_connect',
				'type'    => 'taxonomy_advanced',
				'taxonomy'   => 'brand',
				'field_type' => 'select_advanced',
				'visible' => ['cc_activate', true]

			),
		),
	);

And I'm trying torequest this info as a test:

if (!function_exists('cc_slug')):
    function cc_slug()
    {
    	global $prefix;
	    $terms = rwmb_meta( $prefix . 'cc_connect' );
	    $content = '';
	    if ( !empty( $terms ) ) {
		    $content .= '<ul>';
		    foreach ( $terms as $term ) {
			    $content .= sprintf(
				    '<li><a title="%s" href="%s">%s</a></li>',
				    esc_attr( $term->name ),
				    esc_url( get_term_link( $term, 'tax_slug' ) ),
				    esc_html( $term->name )
			    );
		    }
	    }
	    echo $content;
    }
endif;

I get in var_dump(cc_slug()); NULL

So I really do not understand how exactly Taxonomy Advanced work.

thank you