It's a problem with your configuration. Does the PIX have a globally routable IP address, or is it a provate IP address? If it's private, the the remote router won't be able to get to it, and you'll have to add a static NAT translation on the outside router that maps through to the PIX's address. You can then point the DHCP router to this NAT address and it should work.
Can you run crypto debugs on the PIX and send us the output when the tunnel is attempted to be built?