UART TTL/USB on Mi Router 3g

I am trying to get to u-boot on an OpenWRT router using TTL/USB converters but I am failing. I tried a CP2102 and a Ch340g (3.3V). I can get no data on the serial consoles with either.

I set up the pins as recommended:

TX on the USB -> RX on the router
RX on the USB -> TX on the router
GND USB -> GND on router

I tried inverting TX/RX also to no avail. I have tested in serial consoles in both windows and linux, but no data is transmitted.

I tried a PL2303HX 5V cable also. This one prints a lot of gibberish on the terminal, but I suspect the voltage isn't adequate.

Anyone got any idea?

What terminal program do you use on the PC? Disable HW and SW flow control and make sure you use the correct baud rate (usually 115200). You must use 3.3V adapter and be sure that the physical connection is good - bad ground can cause what you're describing.

Yes, the CP2102 and Ch340g are 3.3V, but no data.

I have tried a few types of terminal programs, screen on linux, putty and TaraTerm on windows. I suspect I am missing something on the hardware setup. How can I troubleshoot the grounding?

Which one exactly?

You may need to solder some jumpers on the router board, check the traces coming to/from the pins.
You can check your cable by interconnecting Tx and Rx and typing something in your terminal.

Mi Router 3g

take photo of router, TTL pins, CP2102 wired together ...

otherwise, we cannot help you ...

serial is only really needed for v2, is that what you have ?