Access S3 bucket via VPC endpoint from EC2 instance in private subnet using boto3

I have a flask application that is running on an EC2 instance in a private subnet. The flask application needs to be isolated with no internet connection and still able to access an S3 bucket - to do so, I have set-up a gateway VPC endpoint for S3. However, the flask application is never able to establish a connection to S3 (via boto3) and ends up timing out. Any ideas as to why this is happening?

Somethings to note:

• I have a few other VPC endpoints configured (all of which are of type interface) which work perfectly fine with the Flask application in this deployment.
• When I SSH on to the EC2 instance and perform the command: aws s3 ls, it works and I get a list of all of the available S3 buckets - which suggests to me that the S3 VPC endpoint does in fact work.
• If was to deploy my EC2 on a public subnet with a public IP the flask application works perfectly and is able to access all aws resources that it is dependent on with no issues.