I'm unable to receive data on the serial monitor of the RECEIVER(arduino)

Please i need help.I’m using XBee S2 one configured as COORDINATOR AT and the other one as ROUTER AT. I’m trying to send a simple data from one Arduino to the next Arduino using XBee. The problem is that i can’t see the transmitted data on the serial monitor of the receiver Arduino but i can see it on the CONSOLE of the XCTU when i connect the ROUTER.

TRANSMITTER
int a;
void setup() {
Serial.begin(9600);
}

void loop() {
a=399;
Serial.println(a);
delay(2000);
}

RECEIVER

void setup() {
Serial.begin(9600);
}

void loop() {
if (Serial.available()!=0)
{
int b=Serial.read();
Serial.println(b);

}
}

Most arduino code or libraries use the XBee in API mode.

Also check that the two radios are associated to each other. Then make sure that the DL and DH are set to the other units SL and SH values.