Can somebody plz help me to solve this problem?

I have 2 Networks and 1 Internet Connection and 1 WAN connection to a big company's network!
I want to replace our cisco router with m0n0wall. The cisco works fine but it is to slow (WAN only
10Mbit/s), not easy to configure, not extendable and toooooo vulnerable!

Network diagram:

Internet
    |
Proxy's
    |
Big Company Network
    |
Router X (I can't configure this)
    |
Our Network 1 
    |
----------
|  WAN   |
|m0n0wall|
|  LAN   |
----------
    |
Our Network 2
    |
Sonicwall
 |	  |
DMZ   Internet


The Hosts in our Network 1 have configured m0n0wall as their default gateway.
On the m0n0wall i have Router X as default gateway on WAN and some static routes for DMZ/VPNs and so
on.
m0n0wall doesn't redirect tcp/udp connections incomming on the WAN interface to the responsible
router!
For example, if a host from "our network 1" wants to connect with telnet to a host in the "Big
Company Network" m0n0wall blocks the connection instead of redirecting it to "Router X"! Ping's are
redirected but tcp/udp connections are blocked!

any ideas?

thx