I ran into consistent troubles getting OpenWRT installed successfully on a Netgear R6700v2. I used the factory GUI to upload the 21.02.2 firmware, which would start running just fine. However, when I tried to reboot the router it would fail to boot. I could use the nmrpflash utility to get a Netgear factory firmware loaded and booted up again, and I tried several things that seemed like they might work - older factory firmware to install from, erasing config nvram before flashing, different OpenWRT releases - nothing seemed to work until I tried the following: I flashed OpenWRT from the factory GUI, then once that had started running, I connected via ssh and did a soft factory reset with the command firstboot && reboot now
After that I could reboot the device and it reliably booted OpenWRT.
If my experience is not unusual it would help to have some note about this on the TOH page for this device.
There are several images for this device. Which one did you flash? If you installed the initramfs image, you need to follow that with a sysupgrade image upgrade (once you are booted into openwrt initially).
I flashed the squashfs-factory image from the 21.02.2 releases downloads.