Your developer installed Webmin. He wanted you to use Webmin to manage a web hosting service, apparently. What your idiot developer should have done is install Virtualmin instead of Webmin and then you would be able to do everything that you need to do to manage a web hosting service in a most straight forward manner.
It was my mistake too, to not have picked up on that incongruency in your first message. I responded by answering your specific questions without first understanding your use case. My bad. I should have asked you what you intend to do with Webmin and when you would have replied that your developer has installed Webmin as a tool for you to host websites, I would have immediately told you that he is making you bark up the wrong tree. It is Virtualmin and not Webmin that your developer should have offered to you.
Well, there is a lesson in this for all of us to learn…