Flashing OpenWrt 21.02.0-rc1 over 19.07.7 gave a warning then network ports stopped working after reboot (no Rx packets).
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WR1200JS
The 4 pin serial port is in the middle of the board.
Serial: 115200 8N1
Serial connection parameters: Coding UTF-8m (receive) Tera Term
PCB Prolific
Pin Wire
1 RED 3.3V (Not used square pin)
2 GRN
3 WHT
4 BLK GND
Recovered with BREED bootloader (through serial console interrupt) on http://192.168.1.1 .
Reverted back to 19.07.7 successfully.