@m-tarek - Technically, only the Pi needs a connection out to the Internet in order to display the modules correctly. Your devices behind your firewall/gateway/router will need to be able to see the Pi in order to display the interface. (Be sure to carefully configure your IP whitelist)
We have a similar setup in my office for the superwide mirror in the lobby. The gateway has a bypass set up for the Pi running one of the Magic Mirror installations (the superwide runs two Magic Mirror installations). Everything behind the gateway is forced to our capture portal except the Pi, which is allowed to bypass the gateway and get an internet connection. Anyone behind the gateway can see the data displayed upon the Pi without having an internet connection yet. There is some other trickery that we do that isn’t relevant here.