Recently, I have a batch of OEM PoE routers that do not have a USB interface. It is just used to study the TTL flash machine.
Then connect the USB to TTL. Note here that some boards will be incompatible with garbled characters. You can try to change the baud rate. If not, just change a USB to TTL board.
The computer uses SecureCRT, serial port connection, there is no character on the connection, then power on the router, the screen starts to display UBOOT, press any key to interrupt, some press TPL interrupt or ctrl + c interrupt, I first flash...