The easiest way with R7800 is to use the Netgear's recovery TFTP flash routine (in u-boot bootloader) to flash the OpenWrt factory.img file. The routine can be triggered with the reset button/hole at boot.
Advice in:
Ps.
The Netgear OEM firmware can also be installed with the same process (and they also give that advice on their site). https://kb.netgear.com/000059633/How-do-I-upload-firmware-to-my-NETGEAR-router-using-a-TFTP-client-on-Microsoft-Windows?language=en_US