Removing dd-wrt firmware and restoring to factory condition

I purchased a Netgear Nighthawk X4S (R7800) last year and my brother put dd-wrt onto the Router and I want to restore it to factory condition with Netgears firmware on it.

I have tried to do the firmware update within dd-wrt firmware and it starts for about 5 seconds and comes up saying updated successful but when the router restarts it still has dd-wrt installed. I have googled how to do it and tried them all but to no success, there was 1 youtube video that stated that you need to download the initial firmware release and use the chk file in the dd-wrt update firmware however the files you download is a disk image file not .chk file, is there any where I can download a .chk file for my router or can somebody suggest another way for me to try.

Thanks