You should be good as is. Check the spec page of your Amazon link, it says 3.3V.
In other news, the offending commit has been located and reverted. I'd wait for new master images from the buildbots before trying to flash them (you can of course build your own).
If you measure between GND and RX you should see the nominal voltage. Between GND and RX you will see ~50% as it is pulling high and low in case anything is listening.
Important note: usually the pins are labeled from the device perspective. So the TXD (transmit data) pin of the adapter must be connected to the RXD (receive data) on the router, and vice-versa.
Below are more details for reference:
https://www.circuitbasics.com/how-to-set-up-uart-communication-for-arduino/
Recovered successfully and went back to 21.02.1 for now. Thanks a lot everyone.
Have you tried 192.168.0.1? This router has a recovery web page. Otherwise you may need to open it and use an UART to activate the recovery page as described in this topic.
linking to my post there,...if the lights are solid on....ur router is recoverable without UART.
You are awesome! You saved me a lot of time and won a burger and 
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Glad it helped!
Sorry for replying late but thanks a lot for this!
Can you explain how you connected it without soldering?
I have this (RPi) serial adapter and also a Pine64 woodpecker serial adapter, but I assume I'd need pins (like RPi's GPIO pins) to connect them, but all I see are the holes.
I have no soldering experience, so if I can do without that, that would be great.
I used a 3 pin header (like the one below, but cut with 3 pins only). I plugged the shorter side (without soldering) to the router board, and to the other side the serial wiring.
Thanks!
Do you know if it's possible to attach an UART (with those pins) and then close the enclosure again?
I don't think so. The enclosure of this device is pretty tight. So unless you drill a hole for the UART wires, I do not believe it will be possible to close the case with the UART wires attached.
Thanks for that very useful info, much appreciated 
The Pine64 UART adapter does have individual wires, which might fit in the existing holes, but it looks like those holes are below the board, which means the chances of success are quite low.
I thought it may be an interesting learning experience, but I'm not willing to damage things for that.
Cheers 
Turns out, it is possible 
The wires themselves probably could go through the ventilation holes, but the connectors you'd have on each end are too big for those holes ... but they do fit through the wall-mount holes!
Right now, I have a 4 pin header, unsoldered, with 3 wires attached to them (GND/TXD/RXD) and then I closed the device up again \o/
My guess is the space is tight which holds it in place. Tight, but (just?) enough.
Looks like I cheered too soon ...
When I initially connected it, I could follow the whole boot via the serial. But I didn't manage to provide input. One should be able to do '4' or 't' to interrupt the boot process or 'x' to start the web recovery, but I wasn't able to trigger that a single time.
I have used a serial connection before and then I'd see a login prompt when the boot has finished, but I didn't see that now.
I'm still on stock firmware as I'm assuming that's needed to fill in (some of) the gaps on the device wiki page.
I opened up my router again to see whether I could improve things, but since then I either saw no boot log or only partially. Whatever I tried since, it's mostly useful. So it looks like my initial attempt really was a lucky shot.
I hope someone can tell me or give me hints what to do in order to make the serial work properly and reliably.
Check if there is a missing zero-ohm resistor in the rx path, something TP-Link loves to do in order to prevent serial console input.
Thanks
Tbh I don't know what the rx path is, but I'm going to a hackerspace tonight and hopefully they do.
But if you have links to learn more about 'rx path', I'd appreciate it.
I did do a search here, but I mostly saw things wrt SQM/CAKE and another one which seems relevant, but I don't understand what it says (and whether it actually is relevant in my case).
EDIT: Router bricked, then not powering up after using self-made serial cable - #8 by anon57995562 seems potentially useful; going to watch the vids.
EDIT2: Not so useful after all
The router's rx path (at least in this sense), is the physical copper trace on the PCB between its serial rx pin (the one where you plug in your usb2serial tx connector) and the SOC itself. This copper trace should be uninterrupted, but TP-Link likes to cut it (by not assembling a zero-ohm resistor), meaning for anything you enter to reach the SOC, you will have to (solder-)bridge that connection to fix the connection.
A high-res photo of your PCB around the serial connector might help to determine that.
