Hello,
If the DDNS service is deactivated, it means that information of the IP assigned to your WAN interface is no longer sent to your DDNS provider. However, if the same IP is still valid, even when the service is disabled, the record is held by the provider and hostname is resolved. When the address changes and the service is no longer running on the router, service provider does not receive IP change and the device can no longer be.
During my testing, device was reachable by its domain even after deleting DDNS instance till its IP did not change.
Best regards,