after opening the router you should find some thing like TX RX GND (VCC dont use this VCC else you may fry your board)
connect your serial connector TX --> RX in Router and RX in Serial connector to TX in router and GND to GND (dont power on the router) there is no need to solder just insert male breadboard pin in to it thats all it will connect
connect your router with CAT5 ethernet cable to your pc
download putty https://the.earth.li/~sgtatham/putty/latest/w64/putty-64bit-0.73-installer.msi 1
find the com port of the serial connector in device manager and enter the com(port number) in putty choose serial in putty with the speed 115200 click open
configure tftpd server with computer ip as 192.168.0.225 , bind to it, rename the openwrt-19.07.2-ramips-mt7620-tplink_archer-c5-v4-squashfs-factory.bin and copy test.bin to tftpd folder
power on router press 4 continuously in the putty console (untill uboot stops for the command )
enter the code below one by one
tftp 0x80060000 test.bin
erase tplink 0x20000 0x7a0000
cp.b 0x80060000 0x20000 0x7a0000
reset
if everyting ok then your device will boot up openwrt if not dont panic because the above code will not flash uboot, just copy paste boot log from putty console here will do my best.