SOLVED: UART connections
Here is an example of 5 pin serial console port connections on a router board:-



If you just want to see the booting process then you only need two connections, GND and TX. 

You can determine which pin is Ground using a multimeter's continuity test function. Find another point somewhere in the router which is known to be Ground, then meter (with router power off!) between it and each pin of the serial port  until you get 0 Ohms.  

Once you have identified the GND pin you can connect the USB/serial adapter's Ground to it. Then connect the adapter's RXD line to each of the other pins in turn and boot the router, until you find the one that sends the bootup message.