How to make backend and frontend comunicate in tha same cluster in ECS?

Hello, people. I've made deploy of my backend and frontend application in ECS. The backend uses Java & Springboot in a docker container. The frontend uses Angular 11 in a docker container. The backend is working: I have tested the endpoint in postman!

But I have a issue in frontend: the APIUrl points to "http://localhost:80/myContext". And when I try to make requisition by the frontend, I receive "connection refused".

I have read that, if I put both containers in the same cluster, frontend and backend could comunicate by "localhost". But it is not working... Please, help.