There is a IPsec VPN Amahi application fro your HDA. This VPN stack requires two port to be forwarded from your router to your HDA. These are

 500 UDP and 4500 UDP

Not TCP.

iOS Setup

Go to Settings > Network > VPN > IPSec, enter the settings below and click on Save.

Description: Amahi IPSec VPN
Server: YOURNICK.yourhda.com (put your Amahi dyndns address, nickname and host)
Account: USERNAME
Password: leave it as "Ask Every Time" or put one if you feel it's safe to leave it there
Use Certificate: leave it as OFF
Group Name: Amahi (with a capital A)
Secret: this is the 4-character you got the when you installed the IPSec VPN app in your HDA
Proxy: leave it as off

Below you can see how the setup area looks in a few Apple iOS devices.

iPad

It should look something like this on the iPad 2, iOS 4.x

It should look something like this on the iPhone 3, iOS 4.x

Currently we only support one IPSec VPN client, a free client from Shrew Soft

Download and install the Shrew Soft Client. (Their Windows VPN Client download page may have more recent versions.)
Open the client, called VPN Access Manager and click on + (Add) to add a configuration
Enter YOURNICK.yourhda.com. Replace YOURNICK with your actual HDA nickname, so that your DynDNS works.

Go into the Authentication tab
Select Mutual PSK + XAuth
Under the Local Identity tab, select Key Identifier, enter Amahi (this is called the Group Name and acts as an extra layer of protection)

In the Credentials tab, the Pre Shared Key should be ready to take the VPN secret obtained in the VPN web page inside your HDA.
The rest of things should work as default
Save

If all goes well, you should be connected and you should see this welcome message:

For easy use, these are the recommended settings
Enable it to be visible in the Tray only, so that it's less obtrusive when it connects
If this is a computer you trust, you may want to also set the "remember the connection username", for even easier use