// necessary to crossconnect 
// TX0(4) <--> Original TX pin, in order to see the output in the Serial Monitor - testing TX0 pin
// RX0(2) <--> TX1(17) for testing RX0 pin
void setup() {
  Serial.begin(115200, SERIAL_8N1, 2, 4); // rx0, tx0
  Serial1.begin(115200, SERIAL_8N1, 16, 17); // rx1, tx1
}

void loop() {
  Serial.println("Loop from UART0...");
  delay(1000);
  Serial1.println("Loop from UART1...");
  delay(1000);
  while(Serial.available()) Serial.write(Serial.read());
  delay(1000);
}