Update: The issue has been solved. It was a faulty reset button. Fixed by soldering.
I've followed the instructions to the letter. I've managed to flash TL-WR902AC V3.0 using the TFTP method without issue. The IP used by the server was 192.168.0.66.
Next, I tried to flash TL-MR3020 v3.2. I've followed the instructions, changing the server ip to 192.168.0.225. No go. It will not download the firmware. Next, I changed the ip to 192.168.0.66. This time I at least got a blip on the second LED indicating that TFTP has been initiated. However, despite this, the firmware is not transferred.
This would suggest that unlike other people with v3.2, the latest pre-installed firmware "1.5.0 0.9.1 v0001.0 Build 211118 Rel.64060n" uses the same .66 ip as TL-WR902AC and might look for a different file than "tp_recovery.bin" (unlikely).
I reflashed TL-WR902AC V3.0 just to check if my setup is ok, and that went through no problem. I also tried a switch in between and disabling Windows ethernet media detection, but no go.
I'm getting "who has 192.168.0.66" on TL-WR902AC V3.0, but on the TL-MR3020 v3.2, it doesn't send any such packets.
Even with the "blip", ithere is no traffic from the MR3020 until it has booted up.
I now noticed that the router doesn't even reset after pressing the button for an extended period. It could be that the reset button has a mechanical failure (the router is brand new out-of-box). Or the latest software installed (not available online, not even searching online the build number) has a bug. I'll know more once my second unit arrives by post. If both exhibit issues, I'll try downgrade in web-gui.