I'd recommend to check firewall between client machine and Kubernetes LB endpoint.

Also, you can try curl <cassandra-url>:9042 from the machine where you run the client to test the network connectivity.

Additionally, explore FixedHostNameAddressTranslator in driver configs, for which you may also be interested in checking the related bug explained in https://github.com/apache/cassandra-java-driver/pull/2007.

Please share your driver configs for the further debugging.