→Routing from the LAN
This of course can be restricted, so you could give a SIM just access to your LAN and not your WAN - i.e. to block internet access whilst allowing them to access your own internal servers.
==Routing from the LAN==
If you are assigning IPs from your LAN to the SIM, then as the [[L2TP]] connection is on a different interface to your LAN -to enable routing from the LAN to your SIM you will need to set proxy-arp=true on the LAN interface.