How to access StackSet instances outputs

Accessing the Output values is limited to the region of the stack.

You can't create cross-stack references across regions. You can use the intrinsic function Fn::ImportValue to import only values that have been exported within the same region.

However, I think the solution to your issue is to approach it from another angle. Create the AWS::GlobalAccelerator::Accelerator and AWS::GlobalAccelerator::Listener in your parent template, then pass the ListenerArn to the StackInstances as a parameter. In the template for the StackInstances, create the AWS::GlobalAccelerator::EndpointGroup referencing the locally created ALB ARN and the ListenerARN parameter.

There is a reference to this in this blog post.

When you use a CloudFormation template to create or update an accelerator, you can specify your resources in any AWS Region.  For example, you can define your accelerator and listener in a CloudFormation template in us-west-2.  You can then define your endpoint groups in different CloudFormation templates in the Regions where your endpoints are.  This simplifies setup, but the tradeoff is that your CloudFormation templates in various Regions aren’t related to each other.  Because these cross-Region stacks are not related, make sure that, when you need to tear down the stacks, that you do so in the correct order. That is, delete the stack with the application before you delete the accelerator stack.