Cognito & IAM setup for unauthorised access, to be used in Unity for Polly

We are trying to setup AWS Polly to use inside a Unity project. Currently we are getting this error: "Invalid identity pool configuration. Check assigned IAM roles for this pool"

We have added Polly permissions to the IAM (find screenshots below):

https://ibb.co/ThL6mcc

https://ibb.co/kqcSTP3

And set up an identity, with unauthorised access, pointing to the IAM containing Polly permissions (find screenshot below).

https://ibb.co/TthT7xs

Before we linked Identity to the IAM, the error we were getting was pointing towards a lack of permissions for Polly, so it appears that code on the Unity side is working as expected.

What are the steps we need to take in order to set up the identity pool configuration to work with unauthorised access? The project we are working on is a site based VR experience, and we are managing all of the machines ourselves, so we have no need for user authentication.