Open your sandbox at https://sandbox-{your-slug}.ringo-sandbox-bah7gweubuecfcct.z03.azurefd.net (same host as public_url from the API).
https://sandbox-{your-slug}.ringo-sandbox-bah7gweubuecfcct.z03.azurefd.net
public_url
For local dev without that hostname, use /{your-slug} on the bare host.
/{your-slug}