How to make two Kubernetes Services talk to each other?

Currently, I have working K8s API pods in a K8s service that connects to a K8s Redis service, with K8s pods of it's own. The problem is, I am using NodePort meaning BOTH are exposed to the public. I only want the API accessable to the public. The issue is that if I make the Redis service not public, the API can't see it. Is there a way to connect two Services without exposing one to the public?

This is my API service yaml:

apiVersion: v1
kind: Service
metadata:
   name: app-api-svc
spec:
   selector:
     app: app-api
     tier: api
   ports:
     - protocol: TCP
       port: 5000
       nodePort: 30400
   type: NodePort

And this is my Redis service yaml:

apiVersion: v1
kind: Service
metadata:
   name: app-api-redis-svc
spec:
   selector:
     app: app-api-redis
     tier: celery_broker
   ports:
     - protocol: TCP
       port: 6379
       nodePort: 30537
   type: NodePort