Here's what you need to do:

1. Add custom error handling for specific HTTP status codes:
   Update the nginx.ingress.kubernetes.io/custom-http-errors annotation to include the desired error codes (502, 504, 500).

   nginx.ingress.kubernetes.io/custom-http-errors: "502,504,500"

2. Specify a custom error page for these errors:
   You also need to configure your NGINX to route these errors to a specific backend that serves your custom error page. You can either update your existing backend application to handle these errors or specify a custom error page service.

Here’s an updated version of your YAML file:

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: isvawrp-proxy
  namespace: isva-iseal
  annotations:
    nginx.ingress.kubernetes.io/backend-protocol: "HTTPS"
    nginx.ingress.kubernetes.io/affinity: "cookie"
    nginx.ingress.kubernetes.io/affinity-mode: "persistent"
    nginx.ingress.kubernetes.io/session-cookie-name: "route"
    nginx.ingress.kubernetes.io/session-cookie-expires: "172800"
    nginx.ingress.kubernetes.io/session-cookie-max-age: "172800"
    nginx.ingress.kubernetes.io/ingress.allow-http: "false"
    nginx.org/hsts-max-age: "31536000"
    nginx.ingress.kubernetes.io/custom-http-errors: "502,504,500"

Make sure your backend application handles the error responses (502, 504, 500) to show the desired custom error page. If necessary, you can define a separate service in Kubernetes specifically for error handling.

You can disable NGINX's default error pages and make sure it passes through the backend application's error pages by tweaking your NGINX configuration file.

Here’s how you can do it:

Pass Errors from NGINX to the Backend: You can configure NGINX to simply forward errors (like 404, 500, etc.) to your backend application instead of showing its own error pages. To do this, update your NGINX configuration with:

proxy_intercept_errors off;
This setting tells NGINX to stop intercepting errors and let the backend handle them.
Ensure Proper Proxying: Make sure you’re proxying the requests correctly to the backend:
location / {
proxy_pass http://your_backend;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
}
Optional - Custom Error Pages (if needed): If you want to handle some errors through custom pages but still pass the others to the backend, you can set specific error pages in your NGINX config:
error_page 500 502 503 504 /custom_50x.html;
location = /custom_50x.html {
root /usr/share/nginx/html;
}

But it sounds like you want the backend to handle everything, so you can probably skip this step.