fwiw, you are not the first to encounter this issue with OpenWRT 18.06.
https://forum.openwrt.org/t/how-to-correctly-change-router-lans-ip-address/21654
Have you tried factory resetting the Asus?
ie. power up Asus. Wait a minute for OpenWRT to load up. Now hold Reset button down for about 10 seconds and release the button immediately when the Power light changes behaviour. Now try and change the LAN IP address with LuCI when the router starts up.
(If you are a Windows user, you can use WinSCP to access and edit the /etc/config/network file)