Add a new user for the phone's VPN connection in the FritzBox under System / Fritz!Box User, e.g. D400Home and set a secure password; you do not need to enter an e-mail address.
Give this user the VPN authorisation, all other authorisations are not required for the VPN connection; the option Access also allowed from the Internet does not need to be active either.
Accept the settings and then display the VPN settings.
Make a note of the user name, the password you have assigned, the server address (fixed external IP or Dyn address) and the IPSec key.
In the web interface of the device, the connection must be activated via the VPN switch on the VPN tab and IPsec Xauth PSK must be set as the VPN type.
Now enter the following data:
When you save the settings, the VPN connection is established. Successful connection establishment is signalled with a key symbol in the top line of the D-400 display.
A key can be assigned to the VPN function to conveniently switch the VPN connection on or off as required; the connection can also be controlled via the phone menu under Settings / Network / VPN.