Application webhook endpoints are blocked on GKE (Kubernetes) - kubernetes

Question

We are implementing kubernetes on GKE.
I got stuck with the following.
The staging application on is hosted on GKE 1.19.7-gke.1500.
We have an application where we have to set up webhooks with applications. When setting up the webhook I get the same error. Namely that the url is not available or does not exist.
The application on kubernetes has a valid certificate. The application works locally (minikube and skaffold) when we use ngrok.
In addition, it is possible to perform an oauth with a third party software. But we cannot activate the webhook because our webhook endpoint is not found.
I looked at the cert-manager documentation, which states that there are known issues between Cert-manager and GKE. They say that this has to do with the settings in GKE, I opened some firewall myself, but without result. https://cert-manager.io/docs/installation/compatibility/
In addition, I have looked on the internet to find out if this problem can be fixed. But I find a lot of information about webhook to be set up for Kubernetes itself but not the application that runs on it.
I think it has to do with a port incorrectly GKE configured?
See our Pod Service setting:
---
apiVersion: v1
kind: Service
metadata:
name: backend
spec:
selector:
app: back-end
ports:
- name: back-end
protocol: TCP
port: 5000
targetPort: 5000
Our Ingress setting:
apiVersion: networking.k8s.io/v1beta1
kind: Ingress
metadata:
name: ingress-service
annotations:
kubernetes.io/ingress.global-static-ip-name: "**.**.***.***"
kubernetes.io/ingress.class: 'nginx'
nginx.ingress.kubernetes.io/use-regex: 'true'
cert-manager.io/issuer: '****'
kubernetes.io/tls-acme: "true"
spec:
tls:
- hosts:
- test.xxxxxx.com
secretName: xxxxx
rules:
- host: test.xxxxx.com
http:
paths:
- path: /api/?(.*)
backend:
serviceName: backend
servicePort: 5000
- path: /?(.*)
backend:
serviceName: client
servicePort: 3000
Updated 03/15/2021
For example we want to set up a webhook for the following endpoint:
"https://test.xxxx.nl/api/v1/webhook/person".
We make a request to PipeDrive to set up a webhook, but then PipeDrive gives the following error: "errors: { subscription_url: [ 'invalid or non-reachable URL' ] } }"
response: [cmd]: kubectl get ep backend
NAME: backend
ENDPOINTS: 10.0.0.21:5000, 10.0.0.22:5000
AGE: 81d