TP-Link Archer C20 v4 is a router with a 5-port FE switch and non-detachable antennas, based on MediaTek MT7628AN + MT7610EN. As compared with the older v1 version, v4 is without any USB ports but has a third central antenna for 5GHz. It also appears to have reduced power consumption.
As of mid-2018, OpenWrt works on it with most features, except for the 5GHz radio. The open-source driver for wireless (MT76) is significantly unstable, causing disconnects and (infrequent) lock-ups.
Configure a computer with static IP 192.168.0.66/24 and a TFTP server. On Debian or Ubuntu you can use either the tftpd-hpa or tftpd server packages.
Rename the downloaded firmware file to tp_recovery.bin and place it in the tftp server's root directory. (If using tftpd-hpa this is /var/lib/tftpboot/; if tftpd, it is /srv/tftp/.) You can test that the file is downloadable with tftp localhost and get tp_recovery.bin.
Connect the computer to one of the router's Ethernet ports while the router is off. Press and keep pressed the router's reset button and power it up. After about 7-10 seconds release the reset button. The power LED will flicker rapidly for ~3 seconds, indicating download of the firmware file.
The router will write the firmware to flash during ~40 more seconds of occasional power LED blinks, and then will reboot by itself, ready for use.