Hi Lukasz.
The “installation” of an SSL certificate has primarily nothing to do with EGroupware.
In a standard installation, a web server is installed as a reverse proxy in order to combine the other services (Collabora Online, Rocket.Chat, Guacamole, …) under one address or to make them accessible.
You must therefore carry out the SSL configuration on the reverse proxy.
Please have a look at this:
However, you have not told us how you installed EGroupware or in which constellation you are operating it. Of course, everything depends on that in this case.
And also where you have to install your certificate.
I have described the use of Let’s Encrypt here:
However, the instructions are for a 19.1 with an Apache reverse proxy! A 20.1 standard installation uses an nginx if you did not specify an apache during the installation.
Maybe this helps you to understand.
If this is a problem for you, our support team will also be happy to help:
We can also help with the migration from e.g. EGroupware 1.8. We do that often, we know our business 
Stefan
