You get DNS error most probably when some other application is using the WiFi in your phone at the same time.
Try to do the following:
- Disable WiFi on the Lumia so it only has a cellular data connection (Settings > System > WiFi).
- Disable WiFi access on the laptop.
- Enable Internet sharing on the Lumia (Settings > System > Internet Sharing).
- Enable WiFi access on the laptop then find and connect to the Lumia access point.
If you do not get internet access on the laptop restart your phone and try to share connection from the phone without security settings (Settings > Internet Sharing > Setup > Security Type).