I tried to change static IP on my router. But it's done now.
According to solution over here, i need to edit /etc/config/network file. So i installed nano, edit static IP within the file and restart the router. Then the static IP is successfully changed.
It is said that there is a emergency revert feature in version 18 that will go back if the web GUI cannot be reached. Change through CLI as the official reference is also no use.
Thanks for replying.