have you seen anyone that sucess flashed one TD-w8980 with uboot first byte 0x03? I have one old firmware that uboot first byte is 0x02, but i cant downgrade to that.
Hi you are skipping too far ahead.
Unless you can make the device display its console output (booting messages/spam) then worrying about flashing is of no use.
You should be able to connect your serial cable to the device while it is powered on and see lots of readable English text instantly. You should also be able to interact with the device via your keyboard, press any key and it will ask you for a username and password. Once you finally see any kind of English you can reboot the device to enter uboot.
At this stage you do not need to worry about uboot or flashing. We are just trying to use your computer like a monitor.
My only guess is maybe you have the pins in the wrong order.