Thank you
Although they sent me a version of that doc, it’s older than the one that you have posted.
It’s really nice on their side that when one client ask a question they reply with an outdated doc.
So that means that the problem is only the SNI and other people might be able to use Let’s Encrypt, and also that those that are hosting two domains on the same server can’t use a HTTPS notification with Redsys, no matter which CA they choose.