Why DrayTek SmartVPN does not work in Windows 10? Well, it will work, but it does not work out of box.
After installing SmartVPN in Windows 10, I set SmartVPN client to use remote gateway and DNS, then I can see VPN connection is green (as usual), and I can get public IP of remote gateway. However, the traffic is not fully routed to remote gateway. From the symptom I guess DNS is not routed to remote.
To fix this problem, I have to disable “Automatic metric” for TCP/IPv4 property of VPN connection, and manually set “Interface metric” to a figure that is smaller than the one of Local connection or Wi-Fi connection, whichever I am using to connect to the Internet.
The theory behind is simple – the smaller metric is, the higher priority of the route. By default settings Windows 10 cannot work out a suitable metric for VPN connection, but Windows 7 can. This is another proof of Windows is getting more stupid.