DLink DSL 300G+ and ATA486 won't work together

Hi everyone,

I have a problem connecting my modem DSL300G+ with the Grandstream
HandyTone 486. I'm using this modem since two years attached to a
Linux box that gets the public IP without problems. I wanted to
connect the ATA486 in between but it gets always the private IP (i.e.
192.168.0.2) from the modem instead of the public one. Then the dhcp
server of the ATA works fine with the PC.

I saw a few post relevant to these two equipments and I wonder if
anyone had this problem before.

Thanks in advance
Jimmi