from Change from Public IP to local IP(192.168.1.100) - Virtualmin - Virtualmin Community
this is just a quick guess. You might need to have both IPs on at the same time to keep access?
On the Virtualmin tab go to Server Configuration / Change IP Address
On that screen you will see where you can change your internal and external IP addresses.
Example: Lets say you were hosting at a provider at 47.5.225.39 but you are shutting down the online host to host it locally on your own network and that address is 192.168.1.100
All you would do is open up that panel I listed above and enter the 192.168.1.100 into the current ip address and then down on the external address you would click the button that says “same as real address” and that will do it.
You would need to restart your server after that.
and look at
How to carry out a server-wide change of IP addresses? - Virtualmin - Virtualmin Community