Use the following yaml file to expose the frontend service via the ingress:

apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  name: simple-frontend-ingress
  annotations:
    nginx.ingress.kubernetes.io/rewrite-target: /
spec:
  rules:
  - http:
      paths:
      - path: /
        backend:
          serviceName: frontend
          servicePort: 80

Use kubctl apply -f simple-frontend-ingress.yaml to connect the nginx-ingress service to the frontend service. As you can see in the rules section, it tells nginx that when someone goes to the path https://<EXTERNAL-IP>/ use in the backend, the service named frontend is listening on service port 80.

Go to https://<EXTERNAL-IP>/ and you should get the following output: