Non-NAT and Secondary IP config.

I've done this several times with other kit (eg. BT Voyager 200,
LinkSys etc.) but I've got a real problem getting a BT Voyager 240 to
work. Here's the scenario:-

Our ISP has issued 8 IP addresses, and the router is setup for non-NAT
with another firewall device behind that. I believe I should set a
secondary IP address on the router to one of the fixed-IP range and
the primary IP address can be anything because in non-NAT mode it's
irrelevent. The router get's its WAN address (frame IP address)
automatically on ADSL login and I can ping that from outside the
network. However I cannot ping the secondary address nor any other
addresses on the fixed IP range.

The firewall behind the router is a VPN gateway and it seems to
connect to its VPN partner on another network ok but I cannot ping
from inside the network to an internal address on the other LAN.

I have done this fine with two BT Voyager 200 units and two LinkSys
firewall endpoints but just cannot get it working with a BT Voyager
240. I suspected routing table problems but don't fully understand
these.

Any ideas, anybody? Can you confirm my use of the secondary IP address
is correct?

Alex.