Access S3 files from Datacenter over VPN to VPC
You need to use...


the Internet, from your data center, to access S3 directly (not via the endpoint) or
an AWS DirectConnect connection to access S3 directly (not via the endpoint) or
an HTTP proxy server inside this VPC to access S3 (via the endpoint)


...because S3 endpoints are not designed to be used across VPN connections.  They are only accessible from instances in the VPC where they exist.


  Endpoint connections cannot be extended out of a VPC. Resources on the other side of a VPN connection, VPC peering connection, AWS Direct Connect connection, or ClassicLink connection in your VPC cannot use the endpoint to communicate with resources in the endpoint service.
  
  https://docs.aws.amazon.com/vpc/latest/userguide/vpce-gateway.html#vpc-endpoints-limitations