Application Gateway ingress controller not working

Question

I using AKS to host some backend apps, these apps are based on asp .net 6.0 containers, and use 80 port to communicate.
I created a ingress which point the host to service and the service related to deployment. And the DNS zone is related the host to the frontend ip address of appgw.
When I using the browser to access the webapi page the status is timed out. and the backend health of appgw show the status is unhealthy with message:

Received invalid status code: 404 in the backend server’s HTTP response. As per the health probe configuration, 200-399 is the acceptable status code. Either modify probe configuration or resolve backend issues. To learn more visit - https://aka.ms/StatusCodeMismatch.  

But if I use VPN to connect to the virtual network and use the pod's ip address to access it and it is succeed.
Weird, right?

The ingress yaml:
kind: Ingress
apiVersion: networking.k8s.io/v1
metadata:
name: ingressxxxxapi
namespace: default
uid: fc1bd1e4-1da9-43b9-add2-d5ba4bc0ad22
resourceVersion: '16194356'
generation: 2
creationTimestamp: '2022-02-28T06:34:15Z'
annotations:
kubernetes.io/ingress.class: azure/application-gateway
managedFields:
- manager: Mozilla
operation: Update
apiVersion: networking.k8s.io/v1
time: '2022-02-28T06:34:15Z'
fieldsType: FieldsV1
fieldsV1:
f:metadata:
f:annotations:
.: {}
f:kubernetes.io/ingress.class: {}
f:spec:
f:rules: {}
- manager: appgw-ingress
operation: Update
apiVersion: extensions/v1beta1
time: '2022-02-28T06:34:16Z'
fieldsType: FieldsV1
fieldsV1:
f:status:
f:loadBalancer:
f:ingress: {}
spec:
rules:
- host: api.xxxx.biz
- http:
paths:
- path: /
pathType: Prefix
backend:
service:
name: xxxxerapi
port:
number: 80
status:
loadBalancer:
ingress:
- ip: 20.24.x.x

Answer 1

I think, you need to define backend like below under spec: The error message indicates the same message

spec:
  backend:
    serviceName: appname
    servicePort: 80

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