Yes thank you you're right I haven't seen the tab.
So I made the wildcard certificate + one for "domain.fr" because "*.domain.fr" doesn't cover "domain.fr" (I say it in case someone is in the same situation than me because as a beginner it is not obvious).
Now everything is working except for the subdmains with "www" like "https://www.subdomain.mydomain.fr".
Unfortunately there is no error message. Just a warning that saying the connexion may not be safe.
Certificate Name: mydomain.fr
Serial Number: 3b4c63ca1f8ab5eam91d7dc8835d50f98a6
Key Type: RSA
Domains: *.mydomain.fr mydomain.fr
Expiry Date: 2021-08-10 20:32:47+00:00 (VALID: 89 days)
Certificate Path: /etc/letsencrypt/live/mydomain.fr/fullchain.pem
Private Key Path: /etc/letsencrypt/live/mydomain.fr/privkey.pem
I don't understand why "https://www.mydomain.fr" works without needed to add a certificate for it and "https://www.subdomain.mydomain.fr" doesn't.