How do I add an action using namespaces, and file separation?

I'm working on a plugin, and I'm trying to get to grips with namespaces and how they work with actions. In a separate file, I have a single function with a namespace declared at the top:

namespace verism\ImageUpload;

function image_upload() {
    ...
    echo 'function successfully called';
    wp_die();
}

In the main plugin php file I'm trying to add an action as follows:

namespace verism;

use function verism\ImageUpload\image_upload as image_upload;

add_action( 'wp_ajax_set_image_from_url', 'image_upload' );
add_action( 'wp_ajax_nopriv_set_image_from_url', 'image_upload' );

I have also tried:

namespace verism;

use function verism\ImageUpload\image_upload as image_upload;

add_action( 'wp_ajax_set_image_from_url', '\verism\ImageUpload\image_upload' );
add_action( 'wp_ajax_nopriv_set_image_from_url', '\verism\ImageUpload\image_upload' );

However, none of these approaches work - when I log the AJAX result to the console, I always get 0.

Note: If I copy the image_upload function into the main file, I can see the correct result, the issue seems to only be when it's located in a separate file.

Can anyone shed some light?

EDIT: This question was deemed too similar to an existing one, and so has been closed, so I cannot accept an answer. However, this question deals with a different issue, namely that of requiring—rather than using—a separate file. User Rup was kind enough to offer the correct answer to the problem, in the comments.