Flashing
https://openwrt.org/toh/hwdata/tp-link/tp-link_archer_c20_ac750_v4
It was
Firmware Version 3.16.0 0.9.1 v600f.0 Build 180124 Rel.58224n Hardware Version Archer C20 v4 00000004
I read something that says TFTPD is needed to upgrade this router's firmware
https://openwrt.org/toh/tp-link/archer_c20_v4#using_tftp_method
Using TFTP method → generic.flashing.tftp
On Linux: Download the …squashfs-tftp-recovery.bin as shown in the table above. 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.
Stopping DHCP
systemctl stop dhcpcd.service
nano /etc/netctl/enp3s0
Description='A basic static ethernet connection' Interface=enp3s0 Connection=ethernet IP=static Address='192.168.0.66/24' Gateway='192.168.0.1' DNS=('192.168.0.1') TimeoutOp=10 SkipNoCarrier=yes
EOF
Remove interfaces IP
ip address flush dev enp3s0
ifconfig enp3s0 down
systemctl disable dhcpcd
netctl start enp3s0
TFTPD
pacman -S tftp-hpa
systemctl start tftpd
cp ~/Dev/router/openwrt-21.02.0-ramips-mt76x8-tplink_archer-c20-v4-squashfs-tftp-recovery.bin /srv/tftp/tp_recovery.bin
UNDOING