How do you access a service that is in another stack?

To do this, all you need is to make it use the same network, then they will be visible. In my case I defined a network called public, that is referred to externally by all my stacks

docker network create -d overlay --attachable public

From there in my docker-compose.yml file I have

networks:
  public:
    external:
      name: public

To access it, just use the service name.