Go to Security > Sign-in Options > More and click Multi-factor sign-in.
For your Synology Account, select 2-Factor Authentication (2FA) and follow the on-screen instructions to complete setup. Synology notes that 2FA uses a one-time code (OTP) after your password, and recommends generating a Recovery Code in advance.
For Secure SignIn, install and enable the Secure SignIn Service package on a Synology device running the newest version. Secure SignIn offers:
- Approve sign-in in the Synology Secure SignIn mobile app
- Verification code (OTP)
Approve sign-in can replace password authentication, or it can be used as the second step of 2FA. For enhanced account protection, use either method as the second sign-in step of 2-factor authentication.
If you use a hardware security key instead, choose Security Key under Multi-factor sign-in.
Sources: [1], [2], [5], [12], [14]