I've tried it on two devices now had similar, but different failures.
The first router displays garbage for the first few seconds then eventually starts producing cleartext. If I reset the device it will immediately switch back to garbage output and remain that way.
The second router is always garbage output with nil change.
So, I bought a Bus Pirate and have finally got around to giving it a go.
I've got in, configured everythign to match my other, actually functional UART adapters but I invariably get junk/garbled output. No combination of hardware/software troubleshooting fixes anything.
Do I have to buy the newer model to get functional UART?