How to access certain WP functions inside custom class, in theme folder

I’m building a custom theme and I want to create a helper class for handling creation of metaboxes in admin panel. I have my class declared like this:

namespace ci\wp;

Metaboxes::init();

class Metaboxes {

    private static $instance;
    private static $post;

    private static $metaboxesPath = TEMPLATEPATH . "/config/metaboxes/";

    static function init() {
        global $post;
        self::$post = &$post;
        add_action( 'add_meta_boxes', [ __CLASS__, 'addMetabox' ], 10, 5 );
    }

    // ADD METABOX
    static function addMetabox($id, $title, $post_type, $position, $priority) {
        if (file_exists(self::$metaboxesPath.$id.'.php')) {
            require_once(self::$metaboxesPath.$id.'.php');
            add_meta_box($id, $title, 'do_'.$id, $post_type, $position, $priority);
        }
    }

[...]

The problem is that when I want to use the addMetabox method, by writing \ci\wp\Metaboxes::addMetabox('front_page_slide_settings', 'Slide settings', 'page', 'normal', 'high'); I get the following error:

Fatal error: Uncaught Error: Call to undefined function ci\wp\add_meta_box() in [...]

I tried several different methods of using add_action inside the class but no matter if it’s a static class, singleton with add_action run at instantiation or a normal class with add_action in constructor, it always results in the said error.

Is there a way to make it work? What am I doing wrong?

You’re actually calling the add_meta_box() function before it’s defined, when you run this directly:

\ci\wp\Metaboxes::addMetabox(
    'front_page_slide_settings', 
    'Slide settings', 
    'page', 
    'normal', 
    'high'
);

You don’t mention where you run it, but it’s too early or you run it in the front-end, where add_meta_box() is not defined.

The add_meta_box() function is defined within this file:

/** WordPress Template Administration API */
require_once(ABSPATH . 'wp-admin/includes/template.php');

Make sure to run your problematic snippet afterwards, e.g. within the add_meta_boxes action, like you do within the Metaboxes::init() call.

The core init action, as an example, fires before that Template Administration API is loaded.