UART1 on AB01: how to get access?

It may be a simple question, but I cannot find the answer…
Apparently, there is an available physical UART interface (on pins number 14 and 15. By the way, I found nowhere how to use such numbers…)

How to access it?

Serial1.begin(9600,SERIAL_8N1, UART_RX, UART_TX);

does not seem to work: it stalls.

Please use Serial.begin();

launches the UART connected to the USB port, for debugging.
I wanted to use another UART. And the pinout shows 2 pins “dedicated to” another UART port.
Serial1 compiles, but when reading the data, it gets stuck.

AB01 has only one serial port.

And this serial port has been connected to the serial chip.