UDP

begin()

Initializes the UDP library and network settings.

// SYNTAX
Udp.begin(port);

If using SYSTEM_THREAD(ENABLED), you'll need to wait until the network is connected before calling Udp.begin().

If you are listening on a specific port, you need to call begin(port) again every time the network is disconnected and reconnects, as well.

const int LISTENING_PORT = 8080;

SYSTEM_THREAD(ENABLED);

UDP udp;
bool wasConnected = false;

void setup() {

}

void loop() {
  // For Wi-Fi, use WiFi.ready()
    if (Cellular.ready()) {
        if (!wasConnected) {
            udp.begin(LISTENING_PORT);
            wasConnected = true;
        }
    }
    else {
        wasConnected = false;
    }
}