Apparently bricked TP-Link RE450v1

Hi,
I know there are quite a few threads about bricked RE450v1s here, and I've read them all!
I happily flashed the 21.0.6 openwrt for this device, then wanted to install travelmate which I couldn't because of the partition space resrtiction.
So I used Attended Sysupgrade (what a great tool!) to make another firmware without PPP (not needed for my purposes) and with travelmate included.
That worked, I downloaded the firmware and installed all okay.
Then I wanted to add qrencode to allow travelmate to produce a hotspot QR code.
I went back to Attended Sysupgrade and added qrencode.
It made the firmware. This time I didn't click on download (I didn't think it was needed) and clicked on install (assuming it would download and install).
However, the device never rebooted and an hour later was still sitting there with no lights on.
When I turn it off or on (with its own power button or the power supply) it flashes all LEDs once, but nothing else.
I've connected serial USB to the pads inside (for some reason the TP3/GND wouldn't solder onto the pad whilst the TP1 and TP2 were relatively straightforward but I think I taped it in place!) and I get gibberish without the ground connected properly and nothing when it does seem to be making contact.
I assume with nothing appearing on serial during reboot that means it's completely dead?

Thought I'd better note it here as a warning to be careful with Attended Sysupgrade on this device.

Ben

GND is often hard to solder because of the big amount of copper that "swallows" the heat from the iron (the GND plane all around). I just heat it longer, until the solder finally melts.

Are you sure about the pinout? Did you try swapping Tx and Rx? Is the baud rate correct?

It is very unlikely that a sysupgrade kills the boot loader. The boot loader partition is usually set read only, so you should get output in any case.

Thanks for the reply -- GND soaking up the heat makes sense.

I'll have another go to attach the serial line when I've got time.

I meant to add, I did try swapping TX and RX (although the docs and pics are very clear, but you never know!), and did try other baud rates, which changed the speed of the gibberish (I'm not a serial port expert but I'm guessing that's expected?).
I'll have another go some time and spend a bit more time soldering the GND to get a better connection, and report back.