Aws autoscaling group in private subnet, private communication?
I am assuming that the backend application has been set up in its own subnet that has been configured for private IP addresses only.  If not I would recommend that you do that as a starter for 10.

With that in place, you could set up an internal application load balancer to work on private only and have your backend ASG use this.
from the AWS docs
https://docs.aws.amazon.com/AmazonECS/latest/developerguide/create-application-load-balancer.html