If you put your PC in the DMZ this is the same as forwarding all of your ports to your PC. Forwarding additional ports will not help. Try changing the MTU settings on your router to 1350 instead of 1500 (assuming you have a DHCP connection). Once you have successfully connected to your VPN and Microsoft Exchange Server and your SAP server, you will then want to find out what ports you need open and take your self out of the DMZ.
To open the ports on this router is simple
go to LAN Config --> then port forwarding
Make up a group name (IE vpn)
Internet side IP address / Airstations internet IP address
In your case select TCP/UDP
port number will be a single port or a block 1720 or 1720-1790
Lan side port (in the example above it would be 1720)