I had a WDR4300 with openwrt 18.06.2. I decided to overclock them using the pepe2k mod method. I flashed the Openwrt 14, keep the modded u-boot inside the openwrt folder, changed the bits to set the cpu clock and flashed it with u-boot-upgrade.
After that i rebooted the router.
Now i have a switch. No wifi, no serial, no Luci, no ssh. It simply behaves like a switch. Take internet from the ethernet cable from the modem and redirect to other ethernet.
Led working, internet working but no chance to access it.
What do you mean by changed the bits to set the cpu clock? Anyway, assuming you flashed correct U-Boot image, power down the router, press and keep pressed the reset button, then power on. It should start with default/safe clocks (400/400/200 MHz). If not, use JTAG or reprogram flash memory with external programmer.