Accessing AWS resources securely using the AWS SDK for .NET

I need to access an AWS S3 bucket and DynamoDB securely using user pools and identity pools. I can log in a user:

        public static async Task<bool> SimpleLogin(string username, string password)
    {
        bool loginSuccess = false;
        string accessToken;
        UserCredentials user1 = new UserCredentials() { Username = username, Password = password };

        //taken from this article https://docs.amazonaws.cn/en_us/sdk-for-net/v3/developer-guide/cognito-authentication-extension.html

        var creds = new BasicAWSCredentials(IAMAccessID, IAMSecretID);

        AmazonCognitoIdentityProviderClient provider =
            new AmazonCognitoIdentityProviderClient(creds, RegionEndpoint.EUWest2);

        CognitoUserPool userPool = new CognitoUserPool(poolID, clientID, provider);
        CognitoUser user = new CognitoUser(user1.Username, clientID, userPool, provider);
        InitiateSrpAuthRequest authRequest = new InitiateSrpAuthRequest()
        {
            Password = user1.Password
        };

        try
        {
            AuthFlowResponse authResponse = await user.StartWithSrpAuthAsync(authRequest).ConfigureAwait(false);

            if (authResponse.AuthenticationResult != null)
            {
                Debug.WriteLine("User successfully authenticated.");
                loginSuccess = true;

            }
            else
            {
                Debug.WriteLine("Error in authentication process.");
                loginSuccess = false;
            }
        }

        catch (Amazon.CognitoIdentityProvider.Model.NotAuthorizedException nae)
        {
            //bad username or password
            loginSuccess = false;
        }

        catch (Exception ex)
        {
            //any other exception
        }

        return loginSuccess;

    }

But, as I understand it, the next step is to use the access token returned to access and identity pool so that I can then access the S3 bucket and DynamoDB. I cannot find any examples that do this.

This code creates Cognito credentials and attempts to log into the bucket:

 CognitoAWSCredentials c = new CognitoAWSCredentials(identityPoolID, RegionEndpoint.EUWest2);

        using (var client = new AmazonS3Client(c, RegionEndpoint.EUWest2))
        {
            var dir = new S3DirectoryInfo(client, "recordings", "924f22fb-2694-4fce-b85a-47b1c59b8466");

            foreach (IS3FileSystemInfo file in dir.GetFileSystemInfos())
            {
                Debug.WriteLine(file.Name);
                Debug.WriteLine(file.Extension);
                Debug.WriteLine(file.LastWriteTime);
            }

            ListBucketsResponse response =
                await client.ListBucketsAsync(new ListBucketsRequest()).ConfigureAwait(false);

            foreach (S3Bucket bucket in response.Buckets)
            {
                Debug.WriteLine(bucket.BucketName);
            }
        }

But, I’m having two problems with this. First, I’m getting a 400 error when creating the S3DirectoryInfo object, and secondly when I’m creating the identity pool credentials, they’re not asking for an access token, which doesn’t make sense to me.