you have to buy a new one, you can practically no longer go for a router distro with as of now around 128MB min RAM requirements on devices with only 64Megabyte RAM.
Compare oem partition map to one in v5 https://openwrt.org/toh/tp-link/archer_c20_v5
Likely you can get mtd info setting OEM log level to maximum, then retrieving after fresh boit.
Serial header is labeled on top right (dont connect vcc, use correct voltage, probably breadboard pins can save from soldering.
Rather use your energy to hustle in a regular job, it will take you a few hours, to earn the money to obtain a supported modern device, then spend time investigating that ones customization.
Ok, type help in bootloader, if it has tftpboot and or bootm command then you should be able to netboot V5 "kernel" image without changing OEM fw.
Run wireshark on tftp "server" just in case there are
There check
ip link - are all mac-s correct
plug cables to see switch ports are in same places
iw reg set BE / JP / US / your place
and check that channel maps and dfs channels are available
power device off to clear radio config and netbootagain.
map smth like vlan 10 to one port and ping around to confirm vlans are isolated.
then vlan 1000 (some switches support 16
combine tagged and untagged on same port, check operation. Some switches fail here, not your fault, just document finding.
Check if leds in luci are correctly assigned ie on/off, blink 300;300ms
Write down here whats still wrong, first we fix non-permanent OpenWrt then proceed to programming.
Check uboot menu if you can tftp mtd contents to your server.
If you dont feel confident just say so and stop, successful netboit saves a week of struggles for the next who tries.
8/64 you have to work out both 24.19-SNAPSHOT and SNAPSHOT, likely it will be discontinued in next major release
EDIT: bootlig says it is very similar to V5, letsplay safe to maki it a supported device and not a dooestop brick.
