Hello, @javierparente

We have a tutorial on how to install Webmin on Ubuntu 18.04 droplet:

https://www.digitalocean.com/community/tutorials/how-to-install-webmin-on-ubuntu-18-04

What you can do is to install Nginx and not Apache. The article has several prerequisites and one of them is to configure the web server. You can follow this tutorial in order to install and configure Nginx:

https://www.digitalocean.com/community/tutorials/how-to-install-linux-nginx-mysql-php-lemp-stack-ubuntu-18-04

and also this one for securing Nginx with Let’s Encrypt:

https://www.digitalocean.com/community/tutorials/how-to-secure-nginx-with-let-s-encrypt-on-ubuntu-18-04

Keep in mind that you need to complete all of the steps listed in the
Prerequisites section of the tutorial in order to install Webmin successfully.

Hope this helps! Let me know how it goes.

Regards,
Alex