VPN Server-side access thru MN-700?

Is it possible to gain VPN access to a server that's
located behind an MN-700 router? Microsoft support told
me to use persistent port forwarding and open inbound
port 1723 (for PPTP - TCP configuration), but no go.

Specifically, my configuration is as follows:

Windows 2003 Server configured for RRAS/VPN, located
behind the MN-700 (running as a router providing local
NAT & DHCP services), which in turn is connected to an
ADSL modem (which has a dynamic IP address assigned by
the ISP, but it seems to be pretty constant).

Suggestions???