I downloaded openwrt-22.03.3-apm821xx-nand-netgear_wndr4700-squashfs-factory.img and installed through the factory firmware upgrade page.
The device now just shows the power on light, and every 30 seconds the internet light, and the red HDD light come on for a second or two then turn off. There seems to be some small network activity when the lights come on, as the link light blinks a bit.
I do not see a wifi signal. I do not seem to see this on my main router's DHCP (I've connected to both WAN and LAN ports to check).
Have I bricked this? Is there any hope of fixing this?
Wifi is disabled by default on a fresh install of OpenWrt.
The best way to see if the device is working is to plug a computer (using an ethernet cable) into the lan port of the WNDR4700. Make sure that there are no other connections (i.e. only the WNDR4700 <-> computer, no other pyhsical connections to either of these devices, make sure wifi is disabled on your computer, too). Set the computer to use DHCP (this is the default for most systems), and then see if it obtains a DHCP lease. If it does, you can navigate to the config page (192.168.1.1) and configure from there.
Looks like the the uboot environment needs to be changed for 22.03 to work. I've not tested this yet since I've not had time to open up mine but the link above should give you the details.
Wanted to follow up here with thanks! A USB UART found that the above issue was indeed the problem, and the proposed resolution brought it back to life. Thank you again @konus and @psherman !