Thanks for the tip!. It looks like a USB powered Arduino Nano cannot handle the long range radios power requirements. I powered the radio module independently and I can send commands succesfully again!
The range has not improved, though, but that's another matter...
Having a deeper look at resistor values is always a good idea.
Indeed, optimal values depend on a lot of parameters. You might find a helpful tool for getting the best values for your setup here: http://www.alciro.org/tools/RS-485/RS485-resistor-termination-calculator.jsp
I buy them on Aliexpress:
If you apply hot air it is easy to take off the white "double sided tape" and the plastic.