Hi,
We have an L2TP configuration example in our Wiki. Some additional info can also be found in the RUT240's manual page on VPN, section on L2TP. Perhaps this will help.
I'm not sure what you mean by "authentication via mobile interface", though. Should the tunnel be established over the mobile interface? If so, all you need to do is set up mobile as main WAN and set up L2TP. By I could be misunderstanding your need, so I will await your feedback.
