char myData[] = "12566272,4294967295";
char *startPtr, *endPtr = nullptr;

void setup()
{
  unsigned long y1, y2;
  Serial.begin(115200);

  startPtr = myData;
  y1 = strtoul(startPtr, &endPtr, 10);
  if ((endPtr != nullptr) && (endPtr != startPtr)) {              // we could parse something
    Serial.println(y1, DEC);                                      // shows: 12566272
    startPtr = endPtr + 1;
    y2 = strtoul(startPtr, &endPtr, 10);
    if ((endPtr != nullptr) && (endPtr != startPtr)) {           // we could parse something
      Serial.println(y2, DEC);                                   // shows: 4294967295
    } else {
      // can't read second entry
    }
  } else {
    // can't read first entry
  }
}

void loop() {}