Backend deployed to heroku not working for React Native app

I was doing this course on React Native and everything works locally but when deploying the backend to Heroku and publish in expo the app is not working… is like the backend never starts, so the frontend can’t fetch the data.
here is the deployment to Heroku
https://done-with-it-backend-ceci.herokuapp.com/api

if you try
https://done-with-it-backend-ceci.herokuapp.com/api/listings
you can see the JSON data

here is the backend on github
https://github.com/Ceci007/done-with-it-backend