@Sp12er, welcome to the community!
- Did you see a big red button that said "Apply Unchecked"?
- If so, did you press it?
- If you are not comfortable on the web GUI, you can login via SSH and edit
/etc/config/network
to alter the LAN IP section - then run/etc/init.d/network reload
2, You can also backup the router's config, edit the same network file, re-compress it and then restore it to the router - when it reboots, it will have a new LAN subnet and IP