To restore to NETGEAR Genie stock firmware:

Select dd-wrt.v24-K3_AC_ARM_STD_128K.bin image first and select erase default configuration after flash.
After DD-WRT boots, flash the R6400-V1.0.0.26_1.0.14.chk file.  Wait until Genie boots up.

Erase settings using reset buton. Just push and hold the button -- Power LED
will change color from white to orange, and after a few seconds will blink,
then release button. Router should automatically reboot.