Error 404 On Redirect Back From Payment Processor

I created a custom payment gateway that directs to a third party to complete the payment. However, on predicting back from the payment processor, I am getting error 404.

The return URL I used is :
$return_url = fn_url("payment_notification.finish?order_id=$order_id", AREA, 'current');

Which gives: ...index.php?dispatch=payment_notification.finish?order_id=101

I attached the error as well. Kindly assist, is there anything I am missing? Anything I should add to make the URL work?