void MainWindow::on_pushButton_WriteID_clicked()
{
    QModbusDataUnit writeUnit = QModbusDataUnit(QModbusDataUnit::HoldingRegisters) ;//specify register type
    writeUnit.setStartAddress(3);//set start address
    writeUnit.setValueCount(1);//set number of data blocks
    writeUnit.setValue(0,4);//set value 4 in index 0 (new slave ID)

    if(auto *reply = ModbusObject->sendWriteRequest(writeUnit,/*device slave ID:*/ 3))//send data unit to slave ID 3
    {
        if(!reply->isFinished()){
            connect(reply,&QModbusReply::finished,this,MainWindow::WriteID);
            qDebug()<<"ok";
        }

        else{
            statusBar()->showMessage("error!",2000);
            delete reply;
        }
    }
    else
        statusBar()->showMessage("error!",2000);
}

void MainWindow::WriteID()
{

    auto reply = qobject_cast<QModbusReply *>(sender());
    if (!reply){
        qDebug()<<"returned";
        return;
    }

    if(reply->error()){
        statusBar()->showMessage(tr("ERROR:%1").arg(reply->errorString()),2000);
    }
    else{
        statusBar()->showMessage("Write Successful!",2000);
    }
    reply->deleteLater();

}

when I Click on Write button "Write successful" is printed but the ID is not changed.