Communication between AWS VPC instances via public IP

We have two AWS instances (Instance A and Instance B) which are running in the same VPC. There is an internet facing service on Instance A which is restricted (via security group) to a subset of IP addresses. Instance A has a DNS entry so the service can be accessed via someservice.example.org.

When trying to access the service from Instance B it works correctly if we used the VPC internal IP address however we cannot seem to get the correct security group configuration to allow this instance access via the public DNS.

We have added the 'default' VPC security group to Instance A but we're still unable to access this service directly. We also have the same problem trying to configure access to Instance A from other VPCs.

I know that we can create a private DNS for the VPC which could solve the problem when we are in the same VPC but this doesn't get around the problem when running in another VPC.