Problem with connecting frontend and backend services through internal load balancer

Question

I'm new to deploying web applications on AWS, but my co-workers are unable to solve the problem so if anyone can help with our problem, it'd be great.

Frontend and backend service seem to be working fine separately on its own, but when I try to communicate between frontend and backend I keep getting ERR_NAME_NOT_RESOLVED error.

• Service discovery are all connected
• All security groups are open

my AWS architecture

I think our architecture is very similar to this if that helps. (https://mohamedwaelbenismail.medium.com/microservices-architecture-deployed-on-ecs-fargate-based-cluster-using-cloudformation-878cb6f90571)

• It only works if we change the internal load balancer to public load balancer allow internet traffic and allow 0.0.0.0/0.
• Status of health check are all 'healthy'