Extending 3rd party plugin class

Hello,

How to extend a 3rd party plugin class with a snippet or in functions.php of my theme? The original class in plugin file I would like to extend looks like this:

global $newsletter;

class Newsletter extends NewsletterModule {
  static function instance() {
    if (self::$instance == null) {
        self::$instance = new Newsletter();
    }
    return self::$instance;
  }
  function __construct() {
    add_action('newsletter', array($this, 'hook_newsletter'), 1);
  }
  function hook_newsletter() {
    $this->logger->debug('hook_newsletter> Start');
    ...
  }
}

$newsletter = Newsletter::instance();

My class in functions.php in my theme looks like this:

global $newsletter;

class MyNewsletter extends Newsletter {
  static function instance() {
    if (self::$instance == null) {
      self::$instance = new MyNewsletter();
    }
    return self::$instance;
  }
  function __construct() {
    parent::__construct();
    $this->logger->debug('my hook_newsletter init');
    remove_action( 'newsletter', array( 'Newsletter', 'hook_newsletter' ) );
    add_action( 'newsletter', array( $this, 'hook_newsletter' ), 20 );
  }

  function hook_newsletter() {
    $this->logger->debug('my hook_newsletter> Start');
  }
}

$newsletter = MyNewsletter::instance();

However when the action ‘newsletter’ is being invoked the original class function is used. What I am doing wrong?

Best regards,
Rob