Well, at this moment thanks to this tutorial I managed to configure Nginx correctly and now when I go to the subdomain of my website, everything works perfectly.
What the tutorial does not mention is how I can add a second domain in the /etc/nginx/sites-enabled/mydomain file since at the end of the tutorial the document already has an SSL certificate that points to a specific IP address and domain.
I have read that it is as simple as "copy and paste below the “server” block but in that specific example there are two server blocks, and if I try to create another file using the same content of that one but changing the IP and subdomain, I get an error when trying to restart Nginx saying there can’t be two files pointing to ::[80]:
Run the certbot client again and this time specify all the domains you want to be part of one certificate.
I hadn’t thought of it and it sounds awkwardly simple, I’ll try it.