$_GET parametters not working in an specific template

WordPress is removing all unknown (not registered) $_GET parameters while processing the request (same is true for $_POST).

You can either register the custom variable, or just go the canonical PHP way by using filter_input(). This will access the original, unchanged GET parameters.

If you are using filter_input(), you have to hook into pre_get_posts to change the fetched resource.

You can also filter query_vars and register your custom variable here. This will make sure it is preserved.

add_filter( 'query_vars', function ( $vars ) {
    $vars[] = 'sereno';
    return $vars;
});

Then you can access it per get_query_var( 'sereno' ) now. You still have to change the request somehow if you want to show some custom content.