Provider or Reseller has a Statement Cycle configured to bill customers on fixed day of month. But Billing Orders still are generated at subscription's next billing date.
To generate Billing Orders on some fixed date of month an option Align Billing Order with Statement Day for Service Plan should be enabled. This option can be changed only for non-delegated Service Plans.
- Log in as Provider or Reseller
- Open some Service Plan at Products > Service Plans on General tab
- Click Edit button, sure that "Align Billing Order with Statement Day" parameter is set to "Yes" and save the changes.
In order to apply this setting to existing subscriptions they should be synchronized with the Service Plan using "Update Subscription Prices" options set. More about subscriptions synchronization is available in Synchronizing Subscriptions From Plan documentation article.