You access Business Manager for on-demand sandboxes with a unified cluster-agnostic host name, so you don't have to remember the cluster that your sandbox belongs to.
To access Business Manager use a hostname that matches
the pattern <realm>-<instanceId>.dx.commercecloud.salesforce.com,
for example, zzzz-001.dx.commercecloud.salesforce.com. It also matches the
domain name of the B2C centralized admin API
server, https://admin.dx.commercecloud.salesforce.com/.