Hi Natalia,
In what case you intend to use anonymous access?
Usually we have anonymous user access for password reset form or a self-registration form.
Normally for the calls to URIs it is required the user to authenticate and the user must have the UME actions "idm_authenticated" and the "idm_authenticated_restapi". So I think there is no way to make anonymous call to the rest API.
Best wishes,
Fedya