During the development phase of a site/domain that I couldn’t get a Let’s Encrypt certificate for, I used the certificate of another site. I just copied the contents of the SSL Certificate, SSL Key, SSL Certificate Authority / Intermediate fields of the other site, to the dev site and checked the Enable SSL for this domain.
Of course the browser issued the warning that the certificate was for a different domain, but upon accepting that warning, I was able to visit the dev site/domain using https. Please bear in mind that this is a quick and (very) dirty approach, because we are exposing the private key of another site to a different one (in Hestia GUI). Only use this approach if you own both sites/domains.
Of course @falzo proposal to get a certificate from gogetssl for a domain that can’t be auto-validated, is way better.