Redirecting to another IP does not work.

I hava a wireless home network which is connected using
a Siemens Gigaset SE567 wireless router.

When I load the main page, I have the ability to redirect
requests to any IP on my network. I have found that
I am not able to do so for some reason.
I have http running on port 80 on another machine
as well as ftp and ssh. I have registered my router's IP
address with dyndns so that I have a website name. But
when ever I attempt to execute a request to port 80,
I get the login screen for the modem. This
happens no matter what machine I am on at the time.
And from what I understand, those who attempt to access it
from outside my network get an "access denied" message or something
like that.

My question is, is there a way to make this work? I used to have
a comtrend DSL modem and a DLink wireless router. When I switched
over to a faster connection, my ISP provided me with this new
modem/wireless router box. But it seems that something is borked
and I have no idea. The help/docs for the box tell me that
I have done everything right. But obviously I have not.

I would appreciate any insight into this issue as my website
is now invisible to the outside world; by the time it's done
it's gonna be really purddy so I'd like for people to be able
to see it.

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