Hello @bartv ,

If you would like to communicate via private IP between a managed database and a managed kubernetes cluster, you need to add both of the clusters to the same VPC: https://www.digitalocean.com/docs/networking/vpc/

However, it is not possible to directly communicate between pods in the different clusters even in the same VPC network. You need to make a service setups like LB or NodePort.
https://kubernetes.io/docs/concepts/services-networking/service/

Coming to the firewall query, unfortunately, we do not support putting LoadBalancer’s behind our Cloud Firewalls. This is a limitation not of DOKS but of the current LoadBalancer product.

However, there is a way to accomplish this by exposing an ingress controller via LoadBalancer. You could then use the plethora of annotations to determine and handle the traffic. You could tell your ingress controller drop or block traffic before reaching your applications using a whitelist. https://kubernetes.github.io/ingress-nginx/user-guide/nginx-configuration/annotations/#whitelist-source-range

I hope this helps!

Best Regards,
Purnima Kumari
Developer Support Engineer II, DigitalOcean