Filestore's S3 driver does not support the default credential provider chain. I have already made changed locally, but pull requests are not allowed on the Github repositories:
https://github.com/FreePBX/filestore/compare/master...clshortfuse:patch-1
It is recommended by the AWS team to use the default credential provider chain. With the new code changes I provided, this would automatically happen if the user does not supply any keys. The change is small, but necessary to work with IAM roles.
See: https://docs.aws.amazon.com/sdk-for-php/v3/developer-guide/guide_credentials.html