Does 192.168.100.0/24 somehow conflict with your WAN?
If not, then you have to change your client IP at the same time. When you change your LAN IP, DON'T HIT APPLY. Go to Services->DHCP Server. It will need to be configured to offer addresses in the new IP scheme. After that, you can go back to the interface and apply the interface changes. You will then need to somehow make your devices release/renew DHCP. You can either do this with something like ipconfig /release then ipconfig /renew, unplugging the ethernet for a few seconds and reinserting, or rebooting.