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