I'm facing exactly the same problem. I'm trying for literal hours now. I tried every possible configuration, put 1K resistor, removed resistor, checked oscilloscope (TX gets sent, I can see bits), tried other adresses, other drivers (5 of them), checked 5VDC, checked EN, checked VM, checked little bridge underneath by diode mode, pulled random MS1 and 2 high/low, bent certain pins, yadda yadda yadda.
In standalone mode it works fine, but UART won't work no matter what I do.
Below an image with the OSC hooked up between GND and PDN_UART (It captured the TX from the RAMPS board when I sent the "M122 I" command) :
Something I've also observed is that MS1 and MS2 get pulled to 5V even though I haven't set a jumper, that's why I said above that I bent them, so that they stay low, but to no avail.
All adresses fail (0, 1, 2 and 3).
Furthermore, I checked if AUX-2 on the RAMPS board is pulled high or low, but after a resistance measure it doesn't seen so.
I also reconfigured the pins.h file, sacrificed the Z endstop (becuase the pins are wasting a Hardware Serial (Serial1)) and put it to Serial1, configured to HW Serial and still nothing.
Lastly, I downloaded a example sketch for driving a simple TMC driver (TMCStepper library) but it also couldn't find the UART interface.
It's driving me crazy!