What do you plan to do with your ISPConfig if it does not have connection to the Internet? Or maybe I do not understand what you mean by "offline"?
Do you really mean offline (desktop system without any network connection) or do you mean an installation in an internal network that ha access to the internet to download software? Installing it on an offline desktop makes not much sense in my opinion and installing it in an internal network is the same as if you install it in a data center, except that you won't get a Let's Encrypt SSL certificate for the system.
