Cannot change domain name

Hi,
I’m facing an ennoying problem.
I wanted to change my domain name from https(.)mysite.com to https(.) mysite-studio.com. I am used to cloning my sites on a local server and never had any problem so I (naively) thought that would be an easy task. I purchased a new domain name through my host, waited for about 40 hours at which time propagation was complete, and started to work.
1. Removed redirections in .htaccess, deactivated security plugin and changed site and home addresses in wp-options: Not OK, both the “old ” and “new” site could not be reached (404 not found on this server).
2. Changed back to the “old” addresses in wp options, OK. re-added redirections + security plugin: OK
3. Tried changing all “old links” for “new links” with “better-search-replace”. Immediately, both “old ” and “new” sites could not be reached (404 not found on this server).
4. Manually changed back to the “old” site and home addresses in wp options, OK. Changed all other links from the dashboard with “better-search-replace” OK. Added redirections + security plugin: OK.
5.
It is as if the new domain is not available although the SSL certificate is active on both domains. I do not see what could be wrong with the database and I did not modify the wp-config (same database) but maybe I am missing something obvious.
According to my host this is a wordpress problem. Maybe but I just cannot figure out what the problem could be. I hope someone can help me understand what is going on. I have no idea why such a seemingly simple change fails. And of course I cannot change the domain name.