I think you have two separate issues. One is that your ISP's firewall might block you from accessing the standard DNS port (53) of external sites, forcing you to use their DNS. This might or might not be a good thing, at least it explains the "Non-authoritative answer" and you might still be able to access the site itself.
The other thing is, why you cannot reach the remote website (port 80). It could be that the site is actually down or overloaded.
The real evil thing would be if your ISP's DNS is giving you not only "Non-authoriative", but false answer. This way they effectively block you from accessing the site. There is workaround for that, you can reconfigure your DNS manually, but it is fairly inconvenient.
I would ask your ISP about it and if you are not happy with their answer, change to an another ISP.