cfn-signal without public IPv4

SOLVED by using IPv6 to call a Lambda HTTP Endpoint from EC2 Userdata which in turn calls cfn-signal on behalf of the EC2. cfn-signal doesn't need to be called from the subject instance, as long as it has the correct paramaters.

Lambda code:

    Type: AWS::Lambda::Function
    Properties:
      FunctionName: !Sub "${EnvironmentName}-MyCfnSignalLambda"
      Handler: index.handler
      Runtime: nodejs22.x
      Code:
        ZipFile: |
          const { EC2Client, DescribeInstancesCommand } = require("@aws-sdk/client-ec2");
          const { CloudFormationClient, SignalResourceCommand } = require("@aws-sdk/client-cloudformation");

          exports.handler = async (event) => {
              console.log(`Received event: ${JSON.stringify(event)}`);

              const region = process.env.AWS_REGION;
              const ec2Client = new EC2Client({ region });
              const cloudFormationClient = new CloudFormationClient({ region });

              try {
                  const { LogicalResourceId, Status } = JSON.parse(event.body);
                  const stackName = process.env.AWS_STACK_NAME;
                  const environmentName = process.env.ENVIRONMENT_NAME;

                  console.log(`Searching for EC2 instance in stack ${stackName}`);
                  const describeInstancesCommand = new DescribeInstancesCommand({
                      Filters: [
                          { Name: "tag:aws:cloudformation:stack-name", Values: [stackName] },
                          { Name: "instance-state-name", Values: ["pending", "running"] },
                      ],
                  });

                  const describeInstancesResponse = await ec2Client.send(describeInstancesCommand);

                  if (!describeInstancesResponse.Reservations || describeInstancesResponse.Reservations.length === 0 || !describeInstancesResponse.Reservations[0].Instances || describeInstancesResponse.Reservations[0].Instances.length === 0) {
                      throw new Error(`Could not find EC2 instance in stack ${stackName}`);
                  }

                  const instanceId = describeInstancesResponse.Reservations[0].Instances[0].InstanceId;
                  console.log(`Found EC2 instance ID: ${instanceId}`);

                  console.log(`Signaling resource ${LogicalResourceId} in stack ${stackName} with status ${Status} for instance ${instanceId}`);
                  const signalResourceCommand = new SignalResourceCommand({
                      StackName: stackName,
                      LogicalResourceId: LogicalResourceId,
                      UniqueId: instanceId,
                      Status: Status,
                  });

                  await cloudFormationClient.send(signalResourceCommand);

                  console.log("Signal sent successfully.");
                  return {
                      statusCode: 200,
                      body: JSON.stringify(`Signal sent successfully for instance ${instanceId}`),
                  };
              } catch (error) {
                  console.error(`Error signaling resource: ${error}`);
                  throw error;
              }
          };

UserData:

                  SIGNAL_PAYLOAD='{ "LogicalResourceId": "MyEc2Instance", "Status": "SUCCESS" }'
                  LAMBDA_FUNCTION_URL="${LambdaFunctionUrl}"

                  curl -6 -X POST -H "Content-Type: application/json" \
                    -d "$SIGNAL_PAYLOAD" \
                    "$LAMBDA_FUNCTION_URL"