403 Forbidden error for Gremlin to AWS Neptune

Thanks a lot for your help in advance

I'm trying to setup my AWS Neptune environment by following the instruction at https://docs.aws.amazon.com/neptune/latest/userguide/get-started.html . The setup seems to be fine, and I could check the status by using the Neptune Notebook install. The status message as:

{
  "status": "healthy",
  "startTime": "Tue May 12 04:24:52 UTC 2020",
  "dbEngineVersion": "1.0.2.2.R2",
  "role": "writer",
  "gremlin": {
    "version": "tinkerpop-3.4.3"
  },
  "sparql": {
    "version": "sparql-1.1"
  },
  "labMode": {
    "ObjectIndex": "disabled",
    "ReadWriteConflictDetection": "enabled"
  }
}

However I can't connect to it via my Gremlin console in EC2 client instance, I'm getting the 403 forbidden error as below:

         \,,,/
         (o o)
-----oOOo-(3)-oOOo-----
plugin activated: tinkerpop.server
plugin activated: tinkerpop.utilities
plugin activated: tinkerpop.tinkergraph
gremlin> :remote connect tinkerpop.server conf/neptune-remote.yaml
WARN  org.apache.tinkerpop.gremlin.driver.Cluster  - Using deprecated SSL trustCertChainFile support
ERROR org.apache.tinkerpop.gremlin.driver.Handler$GremlinResponseHandler  - Could not process the response
io.netty.handler.codec.http.websocketx.WebSocketHandshakeException: Invalid handshake response getStatus: 403 Forbidden
    at io.netty.handler.codec.http.websocketx.WebSocketClientHandshaker13.verify(WebSocketClientHandshaker13.java:226)
    at io.netty.handler.codec.http.websocketx.WebSocketClientHandshaker.finishHandshake(WebSocketClientHandshaker.java:276)
    at org.apache.tinkerpop.gremlin.driver.handler.WebSocketClientHandler.channelRead0(WebSocketClientHandler.java:69)
    at io.netty.channel.SimpleChannelInboundHandler.channelRead(SimpleChannelInboundHandler.java:105)
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:374)
    at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:360)

The IAM role that I assigned to the EC2 instance has the following policy

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [
                "neptune-db:*"
            ],
            "Resource": [
                "arn:aws:neptune-db:ap-southeast-2:<my aws account number>:*/*"
            ]
        }
    ]
}

My neptune-remote.yaml file as below:

hosts: [<my neptune cluster name>.cluster-<cluster id>.ap-southeast-2.neptune.amazonaws.com]
port: 8182
connectionPool: { enableSsl: true, trustCertChainFile: "SFSRootCAG2.pem"}
serializer: { className: org.apache.tinkerpop.gremlin.driver.ser.GryoMessageSerializerV3d0, config: { serializeResultToString: true }}

In order to debug this issue, I have tried to use apache-tinkerpop-gremlin-console version 3.4.1, 3.4.3 & 3.4.6. All have same error response

I have also tried to successfully telnet to above host port as below, so it wasn't a SG or connection issue:

$ telnet <my neptune cluster name>.cluster-<cluster id>.ap-southeast-2.neptune.amazonaws.com 8182
Trying 172.30.1.200...
Connected to xxxxx-xxxxxxxx.cluster-xxxxxx.ap-southeast-2.neptune.amazonaws.com.
Escape character is '^]'.

I have struggle on this for a while, any help/hint will be much appreciated

thanks!