The serial cable required is a “straight through” type. Please note that “proper” PC serial ports tend to work more reliably with Flashwriter than USB-to-serial adapter type serial ports, although if USB-to-serial is the only serial port available it’s worth trying.
To use Flashwriter with serial + Ethernet (recommended method, because Flashwriter can gain lower-level control of the router over serial), set the Communications port number/Interface to the Com port number of the PC’s serial port, and ensure that ‘Use event driven mode’ and ‘Use TFTP’ are both ticked
Then connect serial cable to ASY 0 (Serial port 0) on the router, Ethernet cable (via unmanaged switch between PC and router) into LAN 0 (Eth 0) on router, then click “Load” button in Flashwriter, then follow on-screen prompts.
It’s best to ensure that you have a (non-managed) switch connected between the PC and the router. This is because if you connect the PC directly to the router with an Ethernet cable, the network card in the PC may not initialise fast enough compared to the router boot time when the router is powered on, and Flashwriter may be unable to obtain bootloader control of the router during the router boot process because the PC network card has not had chance to initialise an IP address.
Please refer to the link below