Router to route between private IP address ranges?

I'm setting up a small testbed involving 3 private LANs and a
public/internet segment.

I need to connect two private LANs through a router.

My plan has been to put one LAN on the LAN side of something like a WRT54G
and another LAN on the WAN side of the same router.

So far I've not gotten this to work with either a WRT54G or a D-Link DI-624.

I know that private addresses on the LAN side aren't generally accessible
from the WAN side (while it does work in the opposite direction ..LAN can
access private-addressed WAN addresses). But, is there a way to force
access from WAN to LAN - both having private address ranges like:
192.168.1.1/24
192.168.2.1/24

Thanks,

Fred