Application calling AWS internal load balancer in same subnet is timing out
So what you have actually is this:



As your API ELB is in a private zone it can't be accessed from the internet.
Your frontend in React.js just run in User's browser and not on the UI servers, those server just serve static files.

You have two options, configure your frontend servers to redirect API calls to the API ELB or just update the API ELB to be internet facing.

The usual pitfall of JavaScript apps is forgetting they run inside user's browser and not on the frontend servers as a JEE application would.