how to enable Multi Factor Authentication on AT&T.com?

In the same boat as you, it's crappy on AT&T's part to only offer MFA for mobiles which are on their own network.

To protect your USM Central account, enable multi-factor authentication (MFA). MFA adds extra security because it requires multiple factors to authenticate a user, making it more difficult for an unauthorized person to gain access to the account. In USM Central, MFA provides a layered defense of two independent credentials: what you know (your user account name and password) and what you have (security token on your personal device).

To use multi-factor authentication in USM Central, you must have a mobile device that supports an Authenticator app. AT&T Cybersecurity recommends the Google Authenticator app, which is available for iOS and Android devices. Google Authenticator implements two-step verification services using the Time-based One-Time Password (TOTP) algorithm and HMAC-based One-time Password Algorithm (HOTP) for authentication.

Configure MFA for Your Account

Before you set up MFA for your account, you must install the Authenticator app on your device.

To configure MFA for your account

1. Click the  icon in the upper-right corner of the USM Central web user interface (UI) and select Profile.

2. Click Enable Multi-Factor Authentication.

   The pane expands to provide a unique QR code that is used by the Authenticator app to retrieve a verification code. The code is emailed to you as well.

   

3. Open the Authenticator app on your device.
4. Scan the QR code using the Authenticator app.
5. Click Save in the My Settings window.
6. Click the  icon and select Logout.
7. Click Login.
8. On the login page, enter your user account and password, and click Login.
9. Enter the one-time passcode in the text box of USM Central, and click Verify Code and Login
9. 

Change Your Authentication Device

In the event that you lose or change your mobile device or need to have your MFA reset, please contact AT&T Cybersecurity Technical Support.

Information Source: At&t Cybersequrity

What is USM Central?

I'm an ATT Fiber customer only and I'm using att.com to manage my account 

Exactly, @jegonon   : why are you talking about MFA in USM Central which is probably AT&T Mobile App?   I want to use it from my PC  via Web Browser why logging in via https://signin.att.com/  . I cannot find it there . BTW, my web AT&T account was hacked in and somebody tried to purchase the latest iPhone from there. So, I am trying to enable 2-factors authentication now and this basic security feature is not there...  I know AT&T employees use it all the time to send codes via SMS, but there is no possibility to enable it for web-login. So such thing can happen again!