Cloudformation Property validation failure: Encountered unsupported properties

I'm trying to create a nested stack with the root stack looks like this:

{
    "AWSTemplateFormatVersion": "2010-09-09",

    "Resources": {
        "DynamoDBTable": {
            "Type": "AWS::CloudFormation::Stack",
            "Properties": {
                "Parameters": {
                    "TableName": {
                        "Fn::Sub": "${AWS::StackName}"
                    }
                },
                "TemplateURL": "https://s3.amazonaws.com/my-templates-bucket/dynamodb.json"
            }
        },
        "S3WebsiteReact": {
            "Type": "AWS::CloudFormation::Stack",
            "Properties": {
                "Parameters": {
                    "BucketName": {
                        "Fn::Sub": "${AWS::StackName}-website"
                    }
                },
                "TemplateURL": "https://s3.amazonaws.com/my-templates-bucket/s3-static-website-react.json"
            }
        },
        "S3UploadBucket": {
            "Type": "AWS::CloudFormation::Stack",
            "Properties": {
                "Parameters": {
                    "BucketName": {
                        "Fn::Sub": "${AWS::StackName}-upload"
                    }
                },
                "TemplateURL": "https://s3.amazonaws.com/my-templates-bucket/s3-with-cors.json"
            }
        },
        "Cognito": {
            "Type": "AWS::CloudFormation::Stack",
            "DependsOn": "DynamoDBTable",
            "Properties": {
                "Parameters": {
                    "CognitoUserPoolName": {
                        "Fn::Join" : ["",
                            {
                                "Fn::Split": ["-", {
                                    "Ref": "AWS::StackName"
                                }]
                            }
                        ]
                    }
                },
                "TemplateURL": "https://s3.amazonaws.com/my-templates-bucket/cognito.json"
            }
        },
        "ApiGateway": {
            "Type": "AWS::CloudFormation::Stack",
            "DependsOn": ["DynamoDBTable", "Cognito"],
            "Properties": {
                "Parameters": {
                    "ApiGatewayName": {
                        "Fn::Sub": "${AWS::StackName}-api"
                    },
                    "CognitoUserPoolArn": {
                        "Fn::GetAtt": [ "Cognito", "Outputs.UserPoolArn" ]
                    },
                    "DynamoDBStack": {
                        "Fn::GetAtt": [ "DynamoDBTable", "Outputs.DDBStackName" ]
                    }
                },
                "TemplateURL": "https://s3.amazonaws.com/my-templates-bucket/api-gateway.json"
            }
        },
        "IdentityPool": {
            "Description": "Cognito Identity Pool. Must be created after User Pool and API Gateway.",
            "Type": "AWS::Cognito::IdentityPool",
            "DependsOn": ["Cognito", "ApiGateway", "S3UploadBucket"],
            "Properties": {
                "Parameters": {
                    "AppClientId": {
                        "Fn::GetAtt": [ "Cognito", "Outputs.AppClientId" ]
                    },
                    "UserPoolProviderName": {
                        "Fn::GetAtt": [ "Cognito", "Outputs.ProviderName" ]
                    },
                    "UserPoolName": {
                        "Fn::GetAtt": [ "Cognito", "Outputs.UserPoolName" ]
                    },
                    "UploadBucketName": {
                        "Fn::GetAtt": [ "S3UploadBucket", "Outputs.UploadBucketName" ]
                    },
                    "ApiGatewayId": {
                        "Fn::GetAtt": [ "ApiGateway", "Outputs.ApiGatewayId" ]
                    }
                },
                "TemplateURL": "https://s3.amazonaws.com/my-templates-bucket/identity-pool.json"
            }
        }
    },

    "Outputs": {

    }
}

And I get this error:

2019-06-19 14:45:14 UTC-0400    IdentityPool    CREATE_FAILED   Property validation failure: [Encountered unsupported properties in {/}: [TemplateURL, Parameters]]

It looks like my identity pool stack has some issues with the parameters. But the identity pool stack parameters look like this:

"Parameters" : {
        "AppClientId": {
            "Description": "ID of the App Client of the Cognito User Pool passed into this stack.",
            "Type": "String"
        },
        "UserPoolProviderName": {
            "Description": "Cognito User Pool Provider name passed into this stack.",
            "Type": "String"
        },
        "UserPoolName": {
            "Description": "Cognito User Pool Name passed into this stack.",
            "Type": "String"
        },
        "UploadBucketName": {
            "Description": "Name of the bucket that is used to upload files to.",
            "Type": "String"
        },
        "ApiGatewayId": {
            "Description": "ID of the API Gateway created for the stack.",
            "Type": "String"
        }
    },

The funny thing is: I tried creating each stack on its own, then passed the outputs from them as parameters to the stacks that need those parameters and every single stack was created successfully without any problems.

I've tried to look for what is unsupported but was unable to find any answers.