To change the IP address of your OpenWRT router, you could edit it through the SSH or plug the keyboard and monitor to your custom build router.
Step 1: type in vi /etc/config/network in your console, then press ENTER
Step 2: Press “i” on your keyboard to enable edit mode, then change the IP address to whatever you want.
Step 3: Press “Esc” key, type :wq (w means write, q means quit), then press ENTER to apply.
Last Step: type in reboot to reboot your router, then you can log in to your router with the new IP address.