I have a VPN set up on SBS2003. XP client has a SBS connection manager installed. It works fine (gets connected and allows to access shares) when using a mobile 3G as an ISP on my notebook.
Same when using a home internet connection through an ADSL modem working in DHCP mode, where the client is connected without using a router. When I use a router however (linksys WRT54GL with newest Tomato firmware onboard or WRT54GC working as a DHCP in home network which is also a access point for wifi network) the VPN gets connected and authenticated to SBS, but in is impossible to ping the server nor access it or it's shares.
I have all possible passthroughs (e.g. PPTP) enabled on the router, as well as tried to put my client in DMZ or forward a complete range of ports to it. None of that helped. When a router is between ADSL modem and client it is only possible to connect, but accessing server or it's shares gives not results.