Be aware that although they are portraying xFi Advanced Security as a complete replacement for the Norton software they dropped (see https://www.xfinity.com/support/articles/norton-security-online-sunset), that's a gross exaggeration.
For example, Advanced Security cannot detect threats that arrive in encrypted ("https") data streams. Only software running on the customer's computer or mobile device where the stream is decrypted can do that. xFi Advanced Security is more like supplemental protection to AV software, not a replacement.
Users who relied on Norton should subscribe to the paid version or install another antivirus program or app to take the place of the ones Comcast no longer provides.