A DoS (denial of service) attack is a cyber-attack in which the perpetrator seeks to make a machine or network resource unavailable to its intended users by temporarily or indefinitely disrupting services of a host connected to the Internet. Denial of service is typically accomplished by flooding the targeted machine or resource, in our case Synology NAS, with superfluous requests in an attempt to overload systems and prevent some or all legitimate requests from being fulfilled.
Summing up: A Denial of Service (DoS) attack is different from a DDoS attack. The DoS attack typically uses one computer and one Internet connection to flood a targeted system or resource. The DDoS attack uses multiple computers and Internet connections to flood the targeted resource. DDoS attacks are often global attacks, distributed via botnets.
You can enable DoS protection on your Synology NAS device, but not DDoS.
Note: If your Synology NAS device connects to the Internet via PPPoE, the related DoS protection or VPN pass-through rules must be configured on the corresponding PPPoE interface.