For that you need to use a 3CX FQDN. You cannot use your own FQDN with the 3CX Let's Encrypt integration.
You'll have to backup without license key, uninstall 3CX, release your FQDN in the Customer Portal and then reinstall using the same license. It will ask to choose a FQDN and just choose a 3CX one.
You will have to reprovision and reconfigure apps, deskphones and SBCs for them to use the new FQDN. In the meantime, what we like to do is a forward from the old one to the new one.