Hi,
You could edit the Definition Updates Source configuration which by default points to the Configuration Manager. For the standalone scenario you could enable Microsoft Update and Microsoft Malware Protection Center only. If your clients do not have direct connectivity to the internet the use of a UNC share could also be an alternative, but then requires that you periodically update the share with latest definition updates packages.
Best Regards,
- Proposed as answer by Joyce LMicrosoft contingent staff, Moderator 5 hours 11 minutes ago