Serial Communication of Sony Spresense with ESP8266 E12

I'm trying to create a simple serial communication between my ESP8266 E12 and a Sony Spresense. I have connected the Spre.RX with ESP.TX, the Spre.TX with ESP.RX and Spre.GND with ESP.GND.

Receiver:

byte rcvByte;

void setup() {
  Serial.begin(9600);
  while (!Serial) {;}
  Serial.println("Receiving");
}

void loop() {
  if (Serial.available()) {
    rcvByte = Serial.read();
    if (rcvByte == 'H') {
      Serial.println("High");
    }
    if (rcvByte == 'L') {
      Serial.println("Low");
    }
  }
}

Sender:

void setup() {
  Serial.begin(9600);
  while (!Serial) {;}
  Serial.println("Sending");
}

void loop() {
  Serial.print('H');
  delay(1000);
  Serial.print('L');
  delay(1000);
  Serial.println();
}

Unfortunately, nothing happens. I tried both, ESP as Sender and Spresense as Receiver and vice versa.

It works like a charm when I connect my ESP and a Arudino Uno, in both ways.

Do I have to enable the RX/TX pins with the Spresense somehow? I have tried the pins on the developer board as well as the small board directly. Any suggestions?

I took a quick look into this and my best guess, or tip after checking the code is to try the following on the Spresense side:

Simply change Serial to Serial2.

void setup() {
  Serial2.begin(9600);
  while (!Serial2) {;}
  Serial2.println("Sending");
}

void loop() {
  Serial2.print('H');
  delay(1000);
  Serial2.print('L');
  delay(1000);
  Serial2.println();
}

I have not tested so please do if you can.