expose Speedtouch 510 as router to entire subnet?

I have LAN with Win2003 server and XP clients. and I have another 2003
Server (B) that have software firewall with Website filtering.

the Speedtouce 510 pro is work like router with NAT (I prefer that the
DialUp is on modem),
So I have 2 Subnet:

1 > 192.168.6.0 for all workstation
2 > 10.0.0.0 with connet to one side to medem,and on other side to
Server B

|-------------------|
| 192.168.6.0 | Entire subnet
|___________|
| | | | \
/ \ / \ \
\
\
| 192.168.5.55 |
| 10.0.0.2 | Server B with Firewall
\
\
\
10.0.0.138 Speedtouch 510

How routing Internet trafic of Lan via Speedtouch 510?