Ensure PPTP VPN Service is Enabled in the Remote Access Control Setup.
On the Modem
Disable all other options.
In the PPTP General Setup, select MSCHAPv2 Only, and ensure the MPPE encryption is set to 128-bit.
In the Remote Dial In User area, create a new user, selecting PPTP only. Create a username and password here.
On the Linux PCCheck your underlying (non VPN) connection, and ensure that the MTU is set to 1500:
Click on the network manager in the systray, and select "Edit Connections". Add a New Point-to-point Tunneling Protocol (PPTP).
Suppy the IP address, or resolvable DNS name of the Draytek Modem, and then supply the username and password you set up on the modem in the previous section.
Finally, select Advanced, and ensure MPPE is selected and set to 128 bit.