You set up the subnets in the DHCP server. That's the thing responsible for assigning addresses to its clients.

When I set up my VPN, I discovered very quickly that I couldn't give my main company network the address range of 192.168.0 or 192.168.1 because those ranges were so common among the people who were dialing in. So there would be address conflicts and confusion after they connected. So my main network became 192.168.2, its DHCP server assigned their VPN connections unique addresses in that .2 range when they connected.

Out at the remote end, their local LANs should be set up as 192.168.1 or .0 or 10.something. If the VPN appliance on their side of the WAN connection *is* the DHCP server, then set it to dole out addresses in those ranges.