After logging in to fairapps.net, select "Account" from the dropdown menu in the upper-right corner. In the menu that opens, select "Account Security > Login."
In the lower section, you'll find "Two-Factor Authentication"; there, select "Set up Authenticator App" on the right-hand side. A QR code will then appear, which you can scan on your smartphone using an app like FreeOTP.
You can disable two-factor authentication in the same menu. If you can no longer disable it yourself because your login has expired and you can't access your FreeOTP app, please submit a support ticket.