Images are not loading and return 503 errors.

I’m using a multi-domain plugin, that allows adding multiple domains to a single site and loads the site from end pages with added domain URL:
https://wordpress.org/plugins/multiple-domain-mapping-on-single-site/

Images are loading well on the main domain with Smush CDN active, but they are not loading from an added domain, images are not loading. Images link return a 503 error.