add troubleshooting for DNS

external/access/vpn-access/vpn-access.md

@@ -16,7 +16,7 @@ VPN is used to access the servers that are hosted at the LCSB, but are not reach
 
 ## Download and start the VPN
 
-Once you have reset your VPN Password, please download VPN client from [VPN](https://vpn.uni.lu/)
+Once you have reset your VPN Password, please download VPN client from [VPN](https://vpn.uni.lu/).
 
 **ENTER**
 
@@ -30,3 +30,8 @@ Then depending on your distribution of OS, it will suggest you a link to downloa
 <img src="img/vpn-download-01.png" height="300px"><br/>
 
 
+## Troubleshooting
+
+Despite establishing the VPN connection, if you still cannot reach the internal websites and How-to cards, please make sure that you **do not use a custom DNS configuration**. The DNS configuration can be displayed under the **privacy and security options of your browser** or the **network settings of your operating system**.
+
+<img src="img/dns-settings.png" height="300px"><br/>