When you register a new subdomain for your domain, ie. example.existing.com, you need to add it to your certbot to avoid unsecure connection warnings for your users.
Make sure you have A-records on your DNS-provider that point subdomain towards your main domain:
www.newsubdomain A 3600 123.my.ip.goes.here
Log in to your server and enter
sudo certbot --expand -d www.mydomain.com,www.newsubdomain.mydomain.com
Bang!
The result should be:
Deploying certificate Successfully deployed certificate for www.mydomain.com to /etc/apache2/sites-enabled/thisisyourconf.conf Successfully deployed certificate for www.newsubdomain.mydomain.com to /etc/apache2/sites-enabled/thisisyourconf.conf Your existing certificate has been successfully renewed, and the new certificate has been installed.
Update: The previous certificate is omitted from the server and you can receive notifications about it from Let's Encrypt. You can check your certificate status on your server with:
sudo certbot certificates
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