trojanDNSchanger

Running Norton Security, Version 22.5.4.24.  Had what I thought was network issues and spent 1 1/2 hours on phone / chat with internet provider.  No luck.  I took their suggestion and ran Malewarebytes and found TrojanDNSchanger.  Removed the virus and all okay.  Can someone tell me how this slipped through the cracks at Norton?  I have never, ever had a problem with Norton and use this product exclusively for my clients.  Now I am a bit concerned.