If it is shown as not ready even if it is running with no issue (es: you can ssh into it and all the services are running), could be an misconfiguration of your redinessprobe that I see defined in the YAML file as follow, but I have no clue regarding its behaviour. Consider that accordingly to documentation it should return 0.

readinessProbe:
          exec:
            command:
            - /bin/bash
            - -c
            - /ready-probe.sh

On the other hand, if when you try to access the pod you face some kind of errors, I would suggest you if you didn't do it already to retrieve further information to carry on the troubleshooting running the following commands:

$ kubectl describe deployments
$ kubectl describe pods
$ kubectl describe services

This series of commands could help you in order to understand better what is going on.

Please run them and edit your initial post with the output and I can take a look to them.

To ssh into the pod run:

$ kubectl get pods (to retrieve pod name)
$ kubectl exec -ti PODNAME  /bn/bash 

UPDATE I deployed your YAML files, the pods is running correctly (I believe) what is failing is the probe whose content is the following:

cat /ready-probe.sh
if [[ $(nodetool status | grep $POD_IP) == *"UN"* ]]; then
  if [[ $DEBUG ]]; then
    echo "UN";
  fi
  exit 0;
else
  if [[ $DEBUG ]]; then
    echo "Not Up";
  fi
  exit 1;