Connection problem via DHCP over a bridged modem

Hi

I'm using m0n0wall since years and it never let me down, thanks a lot for
that.

I got a new replacement modem after the old one broke down and can't connect
to the internet since then. The configuration of m0n0 as well as the modem
are the same as before and I'm running out of ideas what to try. The setup
is as follows:

The modem (Zyxel P-660HU-T1) works out of the box/after a hard reset. It is
configured as router and connects via DHCP / ENET ENCAP. This works all fine
and the modem receives a public IP like 188.155.45.*. But as I want to have
the m0n0 connected directly to the internet and not over this router, that
doesn't help.

I put the modem in "bridge" mode, deactivate NAT and DHCP and connect the
WAN port of m0n0wall to the modem. After a restart, everything looks okay,
the m0n0wall receives an IP, but it's an IP address like 10.108.24.*. So it
only gets some (ISP ?)-intern IP and not a public one. Of course I also
can't get a connection to the internet with this address. 

The setup was running around 4 years like that without a problem and since
the replacement of the modem, it stopped. Does anyone have an idea what to
try or where to locate the error? 

Thanks a lot