If TFTP activates, the LEDs on the router usually blink differently.
You can avoid the PC going from disconnected to connected by putting an Ethernet switch between the router and the PC, so the PC always has an Ethernet carrier, and Windows doesn't try to reconfigure the port while the router is booting.
If you only want to flash one of these routers (i.e. you're not trying to develop a new "factory" firmware), hook up serial(*) and flash a WR940 sysupgrade from the bootloader. It should boot being the same hardware. Then treat it as if it were a WR940 from now on.
- From your picture it looks like as TP-Link does on many models, the connection from the RX pin (data into the router) to the CPU has been interrupted. In this case R164 makes that conection and it is missing. You probably will need to bridge the R164 solder points.