The only way that worked for me is use the PPOE part of the A800, and let the Quintum do the login into the modem.
That means that the a800 will be the only user of the ADSL link.
But it works!!!! The border elemnt gets a updated I.P. address every time .
I tried the other solutions, like port forwarding, port masquerading,you name it. This is the only way that i found that works consistenly over months at the time without reseting the unit every few hours.