It's weird
It's not RS232 (signals +12 / - 12 volts)
Have you tried with another serial/TTL adapter?
Sometimes there is a switch so that it is 5 volts or 3.3 volts
Here I have here several adapters CH341 / PL2303 / CP210 ...
I took adapter like this: https://aliexpress.com/item/1726023406.html
I set the switch to 3.3 volts and tried again.
Unsuccessfully. Nothing is output to the serial port of this router.
But the serial port of the old router displays the boot log. There is no interface for flashing.