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.
I don't know if this is a proper photo, but it's likely the best my phone can do:
Does this help?
(Original is 3.2 MB, no idea whether it gets modified when uploading here)
I was wrong
I opened the case up again to take the photos and then tried the serial again with the opened case. And when I pressed the unsoldered pins into a certain angle, I could sent key presses:
U-Boot 1.1.3 (Sep 30 2021 - 10:03:31)
Board: Ralink APSoC DRAM: 128 MB
relocate_code Pointer at: 87f58000
Config XHCI 40M PLL
flash manufacture id: ef, device id 40 18
find flash: W25Q128BV
*** Warning - bad CRC, using default environment
============================================
Ralink UBoot Version: 5.0.0.0
--------------------------------------------
ASIC MT7621A DualCore (MAC to MT7530 Mode)
DRAM_CONF_FROM: Auto-Detection
DRAM_TYPE: DDR3
DRAM bus: 16 bit
Xtal Mode=3 OCP Ratio=1/3
Flash component: SPI Flash
Date:Sep 30 2021 Time:10:03:31
============================================
THIS IS uboot
icache: sets:256, ways:4, linesz:32 ,total:32768
dcache: sets:256, ways:4, linesz:32 ,total:32768
##### The CPU freq = 880 MHZ ####
estimate memory size =128 Mbytes
Press '4' or 't' to break the booting process
Press 'x' to enter recovery web server 0
#Reset_MT7530
set LAN/WAN WLLLL
4: System Enter Boot Command Line Interface.
U-Boot 1.1.3 (Sep 30 2021 - 10:03:31)
MT7621 # cat /proc/mtd
Unknown command 'cat' - try 'help'
MT7621 # help
Unknown command '�LLLLLLLLLLLLH@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@�������������������������������help' - try 'help'
MT7621 # help
Unknown command '��5�5�5�5�5�5�5�5�5�5�5�5�5�5�5�5�5�5�5�5�5�5�5�5�5�5�5�5�5�5�5�5�5�5�5�5�5�55?�����������������������������help' - try 'help'
MT7621 #
I don't know what I can do here, but the RX does work. Probably need to solder it to make it work reliably.
EDIT: help does work if I press the connector 'better' (and constantly).
Hello sir @dsouza thanks for providing a usefull information for the community and i need a help from you, i have archer c6 v3 router and i flash openwrt and router got bricked, reset button, ethernet dead almost nothing is work except power button
The wrong i done is after openwrt flashed ethernet is working as expected but later i flashed tp link official firmware of archer c6 via sysupgrade which at first time provide a checksum error but i forcefully flashed it and router got bricked when i click power button all lights are on
Pls let me know is it possible to fix via uart
Thanks again and hope you will help me by provide a information
See this thread for accessing the UART headers on this specific device: No output from UART headers
Try method 1 on this link.
Hi @AashishAS and @darksky thanks for the reply as i previously said my router ethernet is not working due to brick so https://www.tp-link.com/in/support/faq/1482/ method will not work in my case
Pls let me know what i will do
Thanks and hope you guys will providing me a valuable information
Then UART is the only way. @dsouza can guide you, he helped me when I encountered the same.
Thanks man 
I bricked my router (due to my own doings) and have set up a UART-USB connection using my flipper zero, but it's just reading garbage. I set the baudrate 115200 and 8N1, but none of the bytes read are human-readable.
The power led turns on and stays on for about 10 seconds, then turns off for a second. Then all leds turn green, then one of them turns orange, and then it's back to having the single green one.
im using Archer C6 V3.2 (US)
my issue is a bit more complex,
none of the lan ports work and i have already connected it to UART-USB mode,
recovery from web is not possible as no lan port works
how can i flash the firmware here without using lan, just by directly using serial mode???
For Archer C6U (just like C6 v3 + usb port) web recovery can be activated from uboot console. During boot uboot explicitly say:
Press 'x' to enter recovery web server
Recovery accept stock firmware without any modification.
BTW uboot console command ethon (if I'm not mistaken) init ethernet/switch and network work correctly.
configure access to the serial console correctly, try to start failsafe mode by holding reset and Power at the same time, if you can, download a sysupgrade image for your router, download winscp, access your router through winscp, rename the image to the name of firmware_image.bin, send this same image to /tmp/ and run the command sysupgrade -f /tmp/firmware_image.bin on the serial console with failsafe active and press enter, this will reinstall openwrt again from scratch, I did it like this on mine and it worked.
i know the process of going to recovery, the issue here is that the lan ports are damages so they wont respond to ethon,
lookin for somethin where i can flash the firmware using only serial mode
my version doesnt have openwrt so im not using any, my router is on stock firmware