https://wiki.dd-wrt.com/wiki/index.php/TP_Link_Archer_C9
- Change PC ethernet address to 192.168.0.66 and connect to a LAN port on the router
- Open TFTP and serve archerc9v1_tp_recovery.bin on root
- Power down the router, press and hold the reset button, power on the router and release the reset button as soon as the file starts to transfer