OK, found the thread.
Here's what HOBA wrote:
With Databeestje's help we found a solution:
SSH to pfSense and input the following:
pkg_add -r redir
ifconfig fxp0 192.168.0.2/24
redir --lport 8989 --cport 80 --caddr 192.168.0.1 &
In this example 'fxp0' is the real WAN interface (on which pfSense runs PPPoE).
In the example it gets an IP address (192.168.0.2/24) from the modem's subnet (192.168.0.1/24) assigned and
redir puts a Proxy on port 8989 of the pfSense and forwards it to the modem on port 80
You can access the modem by connecting to any of the pfSense's addresses (e.g. LAN) on port 8989 and get redirected to the modem's WEBGUI.
To access your modem with Telnet simply change the destination port (--cport) to 23 (or 22 for SSH).
In your case you better use the 10.x.y.z network.
just a quick'n dirty translation from German...