Returned a 503 - Deployment was actually successful

I just deployed nginx proxy docker container with the help of using this document fly nginx. I have my web app installed on AWS ECS along with ALB.

We want to provide custom domain ability to our customers. So they can access our app using their own domain as whitelable. I have issues certificates using flyctl command and map the CNAME ( to customers DNS provider (Godaddy). Also add acme_challenge value to the DNS entry in CNAME. The app certificate status is verified in app. I also add origin url (e.g. Did I miss anything here? I dont know origin URL is not reachable using fly nginx proxy?
Can you please help me out here?