L2TP creates a tunnel between two L2TP devices (a client and a server). Since your router is the L2TP client, it will be assigned an IP address by the server. If you want your end device to obtain an IP directly from the L2TP server, you can run an L2TP client on the end device itself. Take a look at the configuration example here.
However, in your case, I would suggest using OpenVPN in TAP mode to enable communication between two sites with the same subnet. A configuration example is available here.