HTTP server over internet

I have at 2 locations A. and B. a Siemens DSL modem (for
internetplusbellen in the Netherlands). In A. I have box with an
integrated HTTP server attached to the modem. The box is an ALLNET
3000RF Mk2 that I want to use to switch the central heating on and off.

Locally in A. this works perfect, but over the internet it doesn't work.
The ALL3000RF is password protected and the password question arrives in
B but then communication stops.

The DSL modem in A. has port 80 open for the ALL3000RF (192.168.2.202
fixed address). I also tried to use this box as exposed server, but that
doesn't make any difference.

In A. I have a second HTTP server on port 81 (192.168.2.201). This is
dreambox DM500 satellite receiver and this one I can operate from B
completely.

Does somebody has any idea how can make that ALL3000RF operative over
the internet?

Regards, Duende