Ah I understand now, the UI is hosted on the Internet by default however any of our packaged applications allow use on a local network without any Internet being required.
Also, you can download the HTML files to your computer and just open the index.html in your browser and it will work without Internet access. This is also true if the site becomes cached in your browser.
For more advanced cases, the files can be self hosted: https://openthings.freshdesk.com/support/solutions/articles/5000164006-using-a-different-server-for-ui-assets
I hope this helps!