The Gateway appliance private LAN subnet must be 192.168.x.x.
You must contact support to change the Gateway appliance private LAN subnet to anything other than 192.168.x.x.
Connect a laptop to the private network (LAN2 or Gb2) on the Gateway appliance,.
Confirm that your laptop received a DHCP address from the Gateway. Its address will be 192.168.x.x.
Open a browser on your laptop.
Go to https://192.168.0.1:10000 to connect to Webmin and log in.
Select System configuration > Network Configuration, and then Network Interfaces.
Select the Activated at Boot tab.
Change the IPv4 setting from DHCP to Static configuration and enter the desired IPv4 address and Netmask on the 192.168.x.x subnet only.
Click Save and Apply.
At this point you will lose your connection with Webmin. Reconnect using the new IP address you just configured on the private network.