Later on, I found out that the hack only works on the TD-W8970A. So I just prepared the commands, that I am gonna use for flashing, and wrote them down in a text editor. I held the four cables (VCC, GND, TX, RX) on the board of the router without prior soldering with one hand and made a copy paste from text editor to terminal with the other hand.
It just took me 1 minute. So, no soldering needed if you just want to make the initial flashing over serial cable.
If you want to use the TD-W8970 as a modem over VDSL or "ADSL with Annex A" I would buy the TD-W8970A and try out the web interface hack.
If you need the router as modem for ADSL with Annex B, you must buy the B Version. In this case flashing over serial is mandatory if the OEM firmware is encrypted (build date 12.10.2013 or newer): https://wiki.openwrt.org/toh/tp-link/td-w8970#serial
Btw, I held 4 wires with one hand on the router's board (VCC, GND, RX, TX) for the serial connection and made a copy/paste of the pre-written commands with the other hand.
Do you have any output (e.g. error) in the terminal?
Under Gentoo I had to add my user to the uucp group to get access. The group might be called differently in your Linux distro:
david@vpc ~/Homepage> ls -la /dev/ttyS*
crw-rw---- 1 root uucp 4, 64 12. Nov 09:44 /dev/ttyS0
crw-rw---- 1 root uucp 4, 65 12. Nov 09:44 /dev/ttyS1
crw-rw---- 1 root uucp 4, 66 12. Nov 09:44 /dev/ttyS2
crw-rw---- 1 root uucp 4, 67 12. Nov 09:44 /dev/ttyS3