Opencart 4 Extension Route throws 404 – PHP

I’m trying to develop an Opencart4 Extension and for some reason it’s throwing a 404 when I try to access it.

I’ve the following structure:

/opencart/extensions/myextension/admin/controller/payment/myextension.php
/opencart/extensions/myextension/admin/template/payment/myextension.twig

and here’s the controller:

<?PHP

namespace OpencartAdminControllerExtensionMyExtensionPayment;

class MyExtension extends OpencartSystemEngineController {

   public function index() {

        $data = array();

        $data['header'] = $this->load->controller('common/header');
        $data['column_left'] = $this->load->controller('common/column_left');
        $data['footer'] = $this->load->controller('common/footer');

        $this->response->setOutput($this->load->view('extension/myextension/payment/myextension', $data));
   }
}

Now when I try to access the http://localhost/admin123/index.php?route=extension/myextension/payment/myextension, I get a 404 page!