If you're installing on a domain controller, you don't need a standalone ATP sensor. You need to configure the detections to detect application installations. With an ATP sensor (non-standalone), you don't need to configure port mirroring.
Reference:
https://docs.microsoft.com/en-us/azure-advanced-threat-protection/install-atp-step5
https://docs.microsoft.com/en-us/azure-advanced-threat-protection/atp-capacity-planning#choosing-the-right-sensor-type-for-your-deployment