Prestashop CDN : set up a deferred loading of static resources via subdomains

[BoboDjinna]

Hello everyone,
I have a shop on which runs Prestashop 1.7.5.2, I would like to set up CND so that the loading time is reduced.

From my hosting (shared), I created 3 sub domains (without any file at the root).
In the "Settings> Performance: Media Servers", I filled in my three subdomains.
Unfortunately, when I go back to the front office, my images are loaded from the CDN but the link (https://www.cdn1.mydomain.com/***.jpg) is not valid so they do not appear.

So would anyone have a solution for me?

Thank you in advance.

PS: My goal is to speed up the loading of my shop.