First I'd triple check if the adapter is fully functional, the wiring (especially GND) and baud rate.
Make sure that the baud rate is 115200, then connect only CP210x's GND and RX to router's GND and TX. After you find the correct lines, connect CP's TX. Fully power cycle the router after every change in wiring. What you're describing can happen if you swap some of the UART lines - in some cases, a good UART adapter can provide enough power to mess with the CPU's UART bus and produce garbage even on powered off board. Another possibility is bad GND.