l2tp into SBS 2008

Trying to VON over both PPP and L2TP into SBS 2008 server. PPP works great
from outside the lan, and l2TP works great inside but won't seem to connect
from the WAN.

I have ports 1701 and 500 UPD directed to the server, as well as 4500 and
1723. The router is a small business linksys/cisco router that has L2TP
passthrough enabled, and Cisco support says there is no reason why it is not
working.

I put the server in the router's DMZ and disabled Windows Firewall, but no
change in connection.

Any ideas?