Send value from BLE Peripheral to iOS device

Question

I am working on an WPF application where it works as a Peripheral. I have another iPhone app which is connected to WPF peripheral and is also able to send dat to WPF. Now I want to send data from WPF peripheral to iOS device. Initially I was able to send the data as part of Static Value of characteristics but now I have to send data multiple times on click of button. It will be like a chat between WPF Peripheral and iOS Client. How do I send these messages from WPF. I have already created following read and write characteristics:

GattServiceProvider serviceProvider;
    private GattLocalCharacteristic _readCharacteristic;
    private GattLocalCharacteristic _writeCharacteristic;
    public event EventHandler<BLEPeripheralManagerEventArgs> RaiseCustomEvent;

    public BLEPeripheralManager()
    {

    }

    public async void InitiateBluetoothConn()
    {
        var peripheralSupported = await BluetoothAdapter.GetDefaultAsync();

        if (peripheralSupported != null)
        {
            if (serviceProvider == null)
            {
                var serviceStarted = await ServiceProviderInitAsync();
                if (serviceStarted)
                {
                    Debug.WriteLine("Service successfully started");
                }
            }
        }
    }

    private async Task<bool> ServiceProviderInitAsync()
    {
        GattServiceProviderResult serviceResult = await GattServiceProvider.CreateAsync(Constants.ServiceUuid);
        if (serviceResult.Error == BluetoothError.Success)
        {
            serviceProvider = serviceResult.ServiceProvider;
        }
        
        var ReadParameters = new GattLocalCharacteristicParameters
        {
            CharacteristicProperties = (GattCharacteristicProperties.Read | GattCharacteristicProperties.Notify),
            ReadProtectionLevel = GattProtectionLevel.Plain,
            UserDescription = "Read Characteristic"
        };
        GattLocalCharacteristicResult characteristicResult = await serviceProvider.Service.CreateCharacteristicAsync(Constants.ReadCharacteristicUuid, ReadParameters);

        if (characteristicResult.Error != BluetoothError.Success)
        {
            return false;
        }
        _readCharacteristic = characteristicResult.Characteristic;
        _readCharacteristic.ReadRequested += ReadCharacteristic_ReadRequested;

        var WriteParameters = new GattLocalCharacteristicParameters
        {
            CharacteristicProperties = (GattCharacteristicProperties.Read | GattCharacteristicProperties.Write | GattCharacteristicProperties.Notify),
            ReadProtectionLevel = GattProtectionLevel.Plain,
            UserDescription = "Write Characteristic"

        };
        characteristicResult = await serviceProvider.Service.CreateCharacteristicAsync(Constants.WriteCharacteristicUuid, WriteParameters);
        if (characteristicResult.Error != BluetoothError.Success)
        {
            // An error occurred.
            return false;
        }
        _writeCharacteristic = characteristicResult.Characteristic;
        _writeCharacteristic.ReadRequested += ReadCharacteristic_ReadRequested;
        _writeCharacteristic.WriteRequested += WriteCharacteristic_WriteRequested;

        GattServiceProviderAdvertisingParameters advParameters = new GattServiceProviderAdvertisingParameters
        {
            IsDiscoverable = true,
            IsConnectable = true
        };
        serviceProvider.StartAdvertising(advParameters);
        return true;
    }

I want to frequently send message from WPF UI to iOS device. Almost all the resources that I could found over internet were on other way i.e, central to peripheral but I want to send it from WPF Peripheral to iOS client.

Edit1: I have tried it like following but it doesn't work.

public void SendMessage(String toSend)
    {
        //_writeCharacteristic..NotifyValueAsync()
        _ = _readCharacteristic.NotifyValueAsync(CryptographicBuffer.ConvertStringToBinary(toSend, BinaryStringEncoding.Utf8));
        _writeCharacteristic.NotifyValueAsync(CryptographicBuffer.ConvertStringToBinary(toSend, BinaryStringEncoding.Utf8));
    }

I don't get anything in my iOS app.