Unfortunately, on our Grid environments we do not offer a way to allow for other servers to directly connect into our infrastructure. Only containers with a relationship defined within a project can interact with the database containers.
While we do offer a way to connect to database containers via an SSH tunnel from the platform CLI tool, it is designed for developers to interact with the database and not for production servers to communicate with each other.
For more information on this feature, please see the following page in the Platform.sh documentation:
https://docs.platform.sh/development/local/tethered.html#ssh-tunneling