After installing openwrt on a Netgear R6260 and then attempting to revert back to the factory firmware, the router bricked and is unresponsive. Here are the details.
I loaded the factory to openwrt installer bin openwrt-21.02.0-ramips-mt7621-netgear_r6260-squashfs-factory.img
Then I installed the latest full openwrt bin openwrt-21.02.0-ramips-mt7621-netgear_r6260-squashfs-sysupgrade.bin successfully.
I installed DDNS from opkg and got Luci syntax errors when I tried to go to system->DDNS.
After several hours, I was unable to figure out how to enable Telnet so I decided to go back to the factory firmware. I uploaded the factory bin file from the Netgear website - R6260-V18.104.22.168_1.0.1.img . I got an error message saying it wasn't a openwrt bin, but that if it was a factory bin, then hit Force upload. I did and it bricked the router.
Router is totally bricked and will not respond to TFTP.. This is obviously a critical bug. If you have a Netgear router, this version will not let to revert without bricking.