I reckon a simple solution is to change the name of the existing function that is called loop() to showMessage() and create a new function called loop() with this content
void loop() {
if (millis() - prevShowMillis >= showInterval) {
prevShowMillis += showInterval;
received = millis();
showMessage();
}
}
and at the top of the code you will also need to define the variables
unsigned long prevShowMillis;
unsigned long showInterval = 5000; // show every 5 secs
...R