I read a previous post as i was attempting to do the total opposite. From what I understood from the UBNT employee is that the tunneling is handled on the client end (L2TP over IPSEC). What I ended up doing was using CMAK to create the connection profile and unchecking "Make this connection the client's default gateway". But if I understand what you are wanting/needing, you will want to make sure that IS checked on the VPN profile.
Additionally CMAK will let you add routes as well if needed.
https://technet.microsoft.com/en-us/library/cc771679(v=ws.11).aspx
I have tested and this works with both Windows 7 and 10 (side note, you don't have to to it from a server as you can add the feature in add/remove programs on WIn7/10. Only cavaet is that if you have 32-bit clients, you will need to create the connection profile from a 32-bit computer.
Here is what I used as a guide for setting up my L2TP VPN (modified of course.)
https://help.ubnt.com/hc/en-us/articles/204959404-EdgeMAX-Set-up-L2TP-over-IPsec-VPN-server
