I struggled with this for a while - it seemed the only option to access google or facebook was to use DPN full route. Even if I assigned google.com to the smart domain route list, I couldn't access google.com when on smart route.
Finally I figured out, adding a DNS server manually to my wifi connection (e.g. 18.104.22.168) would let it work properly. It seems DNS goes normally to your local DNS server, so if this has certain sites blocked, it's not going to work unless specified manually