Trouble setting-up in a new environment:

I'm using 2 identical servers, running NW5.1 and BM3.7 both with the
servicepacks, and eDir 8.6.2
I've setup the machines to work in a site-2-site VPN via cablemodems
IPaddress is from ISP) and it works fine.

A few days ago we changed ISP providers and here the troubles come;
ADSL and
The ISP's IPaddress is delivered at the modem and is NATted to the
BM3.7 box
using a 10.x.x.x IPaddress.

Try to picture this in mind:

68.x.x---(internal network)

-How to setup this VPN, ... I don't think this is possible without
the router/modem in a bridge mode.
-Can a VPN be setup over a NATted envrionment?

Anyone a good idea or document how to set this up? (if possible)