You can find the scanner at the URL tiss.tuwien.ac.at/events/scanner.xhtml, opens an external URL in a new window. If you log in as usual with your standard access data, the scanner is immediately available to you. There is no need to install an additional app on your smartphone. You can simply bookmark the URL and display the link on your smartphone's home screen for quick access (e.g., option in Chrome or Brave browser - "Add to Home Screen"). The biggest advantage of TISS scanner is that login is necessary only every 14 days, so you can register your access even more conveniently. If you do not use the TISS scanner for 7 days, it is also necessary to re-authenticate for security reasons. Another advantage is the built-in verification of the scanned QR code, so URLs of false or fake QR codes will not be called in the first place.
P.S.: A tip for all iOS users so that the camera does not ask for permission every time: stackoverflow.com/a/57368061, opens an external URL in a new window