VPN works on internal LAN, but not externally

Question

I'm running Mac OS X Server 10.6.7 on a Mac Mini and I'm trying to set up VPN so that I can administer the server from outside the LAN.  The problem is that the VPN connection works on the LAN, but I cannot connect from outside. I've tried this from an iPhone tethered connection and from my client's corporate network.
I've read some posts on this so I'll summarize what I've tried:
L2TP and PPTP configured in Server Admin
Server Preferences > VPN Service ON > IP range 10.0.1.210 to 10.0.1.220 with Shared Secret set
Client side L2TP defined as:
Server Address: server.domain.com
Account Name: <myshortname>
Authentication Settings:
Password: <myaccountpassword>
Shared Secret: <same as one set in VPN Service
Client side PPTP defined as:Everything the same exceptEncryption: Maximum
Airport Extreme port settings:
L2TP
UDP: 500,1701,4500
IP: 10.0.1.50 (the server static IP, of course)
PPTP
TCP: 1723
IP: 10.0.1.50
I'm not sure how to set protocols. Maybe someone can explain that piece that I've read elsewhere.
There's something particular about my landscape which I'll describe. I run a DSL connection to a Linksys router for SLPPP purposes (anti-throttling using Tomato MLPPP firmware). This router has an Airport Extreme in its DMZ so that the AEBs can manage the NAT for the LAN. The server sits behind the AEBs.
I have no issues with open port checks on other services that are port-forwarded through the Airport only so I know the Linksys isn't getting in the way.
Any assistance is appreciated.

Answer 1

Yes, that's right, except the Mac Mini is connected wirelessly (I know, not ideal for a server).
The Linksys has the static IP given by the ISP.
server.domain.com has 10.0.1.50, the static IP given by the AEBS.
When I set up the server, I left NAT on the AEBS and decided to manage its server-specific settings via Server Preferences. It seemed less disruptive to me at the time, as though I was adding a server to the existing network instead of replacing the entire network. I also wanted to retain the usage of the Guest Network feature in the AEBS and that means the AEBS has to manage the DHCP.
Not all computers here are meant to connect to the business network. They all connect to the Internet through the AEBS (I believe I set the server to manage the DNS though); however, mine logs into the Mac Mini network.
I haven't tried connecting directly to the DSL. It's difficult right now as the server has to be in a different location then the phone line. I wouldn't have a monitor there to administer the network changes. I plan on changing the phone line location during the pending renovations though.
For a one man operation with a Mac server set up in a home office, what arrangement would you recommend? We have 2x laptops (1 for business, 1 other), 3x mobile devices, and a server that doubles as a media centre.
Thanks