Problem relied in not properly installed and configured nginx ingress controller. By installing it through Helm and deleting all references to traefik to avoid LoadBalancer collision and setting nginx.org/websocket-services: server-service and/or nginx.ingress.kubernetes.io/websocket-services: server-service, we successfully established 101 Switching Protocols with the client.

As the ingress did not have an address, therefore it was never being routed.