Hi all,
Thanks for investing time to this problem and i have also just tested the problem myself. After some debugging, i have figured this is a big mistake on our half. The pinout diagram was incorrectly labelled and caused all the issues. I have updated the pinout digram on the wiki and bazaar page. Please also check below image for reference.
And you were correct, RX and TX on the 4-pin header refer to Serial2 in Arudino, and RX and TX on the 28-pin header refer to Serial1 in Arduino. For more, the USB serial is just Serial.
Sorry for the problems caused and thanks again for spotting out.
Anson