How to fix No route to host: connection refused to load balancer in Kubernetes

gcloud compute firewall-rules create allow-gcp-health-checks \
  --allow tcp,udp \
  --source-ranges 35.191.0.0/16,130.211.0.0/22

gcloud compute firewall-rules list --filter="name~'gcp-health' OR name~'load-balancer'"

kubectl get svc <service-name> -o yaml | grep -A5 ports:
kubectl get pods -o wide
kubectl logs <pod-name> | grep -i "listening on port"

spec:
  ports:
  - port: 80           # external port
    targetPort: 8080   # container port
    protocol: TCP

apiVersion: v1
kind: Service
metadata:
  name: my-service
spec:
  type: LoadBalancer
  selector:
    app: my-app
  ports:
  - port: 80
    targetPort: 8080
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-app
spec:
  template:
    spec:
      containers:
      - name: app
        readinessProbe:
          httpGet:
            path: /health
            port: 8080
          initialDelaySeconds: 10
          periodSeconds: 10

kubectl run -it --rm debug --image=gcr.io/cloud-builders/kubectl --restart=Never -- bash

# Inside the pod:
curl -v http://<service-cluster-ip>:<port>/health
nc -zv <pod-ip> <port>

gcloud compute backend-services list
gcloud compute backend-services get-health <backend-service-name> --global

# Check instance groups:
gcloud compute instance-groups list
gcloud compute instance-groups get-named-ports <ig-name> --zone=<zone>

kubectl get networkpolicies -A
kubectl describe networkpolicy <policy-name>

apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: allow-lb
spec:
  podSelector:
    matchLabels:
      app: my-app
  ingress:
  - from:
    - namespaceSelector: {}
    ports:
    - protocol: TCP
      port: 8080

kubectl get svc <service-name> -o yaml | grep -A10 annotations

apiVersion: v1
kind: Service
metadata:
  annotations:
    cloud.google.com/load-balancer-type: "Internal"  # if internal LB
spec:
  type: LoadBalancer
  sessionAffinity: ClientIP  # optional

# Delete and recreate the Service
kubectl delete svc <service-name>
kubectl apply -f service.yaml

# Or patch to force update
kubectl patch svc <service-name> -p '{"spec":{"clusterIP":"None"}}' --type merge
kubectl patch svc <service-name> -p '{"spec":{"clusterIP":"<original-ip>"}}' --type merge