Thanks a lot, Cattledog, now my board seems read something.
Before I followed this example:
that defines RX/TX inverted in comparison your suggestion:
UART mySerial(digitalPinToPinName(4), digitalPinToPinName(3), NC, NC); //4 is tx, 3 is rx that runs fine.
I still have some issue in my code, but I'll update the result