Routing Gateway to Private ALB

I have a gateway server running on Fargate ECS. I am unsure on how to set-up the routing url for the subgraphs.
In the Apollo docs it says that the sub graphs should not be public facing. If they are running behind a private ALB how do I allow the Gateway to route to the subgraph. What do I use for the routing URL?

The Gateway Server momentarily works then crashes.

Here is the source code. Any help would be amazing!