I updated the firmware of my kimax U35WF router "openwrt-19.07.1-ramips-mt7620-kimax_u35wf-squashfs-sysupgrade" and now when I go to the router's web page, it asks for the user login as root and the password should be blank. This one does not work. Some solution with some program without having to destroy the device.
First try to access your device via ssh, if that doesn't work, https://openwrt.org/docs/guide-user/troubleshooting/failsafe_and_factory_reset should be at your disposal.