How to connect RDS MySQL db using IAM assumed role in Python

I have service running in AWS account A1 and that has the AWS IAM role R1. My service is attached to a service account that has access to role R1.

Now I have an RDS MySQL cluster setup in a different AWS account A2 and that account has a role R2 which has access to connect to RDS cluster.

DB connect permission looks something like below in R2

{
    "Statement": [
        {
            "Action": "rds-db:connect",
            "Effect": "Allow",
            "Resource": "arn:aws:rds-db:*:<accountid>:dbuser:cluster-avcd/<user>"
        }
    ],
    "Version": "2012-10-17"
}

The R2 role also have trust relationship with IAM role R1, so that R1 can assume R2

I am using Python (in my service which is running in account A1), to connect to the RDS MySQL database using IAM auth by assuming role R2. I had used the code below

#to generate db auth token below is the code
    def generate_auth_token(self,username,aws_arn,region,db_host):
        credentials = self.assume_role(aws_arn)
        rds_client = boto3.client(
            'rds',
            region_name=region,
            aws_access_key_id=credentials['AccessKeyId'],
            aws_secret_access_key=credentials['SecretAccessKey'],
            aws_session_token=credentials['SessionToken']
        )
        token = rds_client.generate_db_auth_token(
            DBHostname=db_host,
            Port=3306,
            DBUsername=username
        )
        return token


#Using above token to connect to db in below code
AwsWrapperConnection.connect(
                connect,
                host=host,
                port=port,
                database=db_name,
                user=username,
                password=token,
                ssl_disabled=False,
                client_flags=[ClientFlag.SSL],
                plugins='iam',
                wrapper_dialect='aurora-mysql'
            )

I am getting this error "access denied "Error occurred while opening a connection: 1045 (28000): Access denied for user 'db_user'@'{ip}' (using password: YES)"

I feel that somehow the way I am using assumed role cred in AwsWrapperConnection.connect method is not right when using IAM plugin, I have tried lot of ways from the internet but none of them worked.

What can I try next?

A role R1 already have permission to assume Role R2 in account B.

{

"Effect": "Allow",

"Action": "sts:AssumeRole", [Not DB connect it needs assume role in account B role R2]

"Resource": "arn:aws:iam::<Account-A2-ID>:role/R2"

}