I would like to install OpenWRT on this router.  I tried using the latest snapshot image from 26-Aug-2021, openwrt-ramips-mt7621-tplink_archer-a6-v3-squashfs-factory.bin in the WebUI but it gave an "Invalid File Type" error.  I tried using the TFTP method mentioned on the OpenWRT Wiki but that too did not work.

How should one install OpenWRT on this hardware?

I tried this:

1. Ethernet cable to Port 1 on the device and to my Linux box (static network config using 192.168.0.66).
2. Hold the reset button in on the device while powering the device up.

I can watch my LAN link light come up/eth0 comes up for a short period of time approximately 5-10 seconds into the boot but I am unable send openwrt-ramips-mt7621-tplink_archer-a6-v3-squashfs-factory.bin to the device.  My atftp command keeps timing out. I think the router continues to boot because I can watch the link light on my wired connection go out.

What am I doing wrong?

atftp --trace --option "timeout 1" --option "mode octet" --put --local-file openwrt-ramips-mt7621-tplink_archer-a6-v3-squashfs-factory.bin 192.168.0.1

I see now that despite ordering V3 of this hardware, I in fact, received V2.  I was able to flash the correct image just fine.