When you add a new domain into Virtualmin, an SSL certificate should be automatically fetched from Let’s Encrypt, and automatically renewed every three months.
It is possible that the automatic renewal is not occurring, due to a known Virtualmin bug. The bug should be fixed soon after an upcoming update.
In the mean time, if your website’s SSL certificate is expiring or expired, simply do a manual renewal from the menus:
Virtualmin => your domain => Server Configuration => SSL Certificate => Let’s Encrypt => Request Certificate
It should take only a few seconds and will add three months to the lifetime of your SSL certificate.
Everything else in the above menus can usually be left at their default settings.