How to connect a node to a mobile application?

Hello everyone, I am developing an application on Flutter in Dart, using an external API for algorithm, this is the library used: https: //

I am trying to use kmd to get new addresses but every time I try I get connection errors like: SocketException: OS error: connection refused, errno = 111, address = localhost, port = 57838 # 0

I understand that I am doing something wrong with the connection to the node and the http communication ports but I don’t know how to set them to make them work correctly.
For the node I use docker sandbox.
This is the code from which I get the error:

Future<String> createNewAddress() async {

    final walletRequest = (CreateWalletRequestBuilder()
      ..walletName = 'wallet'
      ..walletPassword = 'test'
      ..walletDriverName = '')

    final handleTokenRequest = (InitWalletHandleTokenRequestBuilder()
        ..walletId = 'wallet'
        ..walletPassword = 'test')

    final keyRequest = (GenerateKeyRequestBuilder()
      ..displayMnemonic = true
      ..walletHandleToken = 'aa')

      final response = await algorand.kmd.createWallet(createWalletRequest: walletRequest);

      final handleToken = await algorand.kmd.initWalletHandleToken(initializeWalletHandleTokenRequest: handleTokenRequest);

      final address = await algorand.kmd.generateKey(generateKeyRequest: keyRequest);

      return address.toString();

Thank you so much for anyone who has the time and willingness to listen to me

This is a duplicate. See Connection refused, errno = 111, address = localhost, port = 57838 #0 - #4 by fabrice

Furthermore, if you are developing a mobile app, you most likely do not want to use kmd.