No SSL on Subdomain

I have here a domain and a 10M mini webspace (perfectly sufficient). This already over 1 year and have never managed to reach the domain via https://www.example.com only with error (connection is not secure). However https://example.com works without problems. SSL and Letsencrypt run on example.com
The DNS configuration is the worst I have ever experienced. The technical support does not care about the problem and refers to the help pages Here you get no help to a specific problem very sad.
Does anyone here have a clue how to solve the problem, or what exactly I need to enter in the DNS configuration?

Hello @nullbyte,

In order for the community to give you feedback, I invite you to provide the domain name concerned.

^FabL

Sometimes there is an error and you have to do it again. Firstly run SSL in a host of your domain (I assume you did it) and after both in domain and subdoiman. I reccomend you to set a redirect 301 from http to https and after from www.example.com You can do it in .htaccess file using this comend:

RewriteEngine On
RewriteCond %{HTTPS} !=on
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

RewriteEngine on
RewriteCond %{HTTP_HOST} ^www.
RewriteCond %{HTTPS}s ^on(s)|off
RewriteCond http%1://%{HTTP_HOST} ^(https?://)(www.)?(.+)$
RewriteRule ^ %1%3%{REQUEST_URI} [R=301,L]