These are the only settings required:

Vigor 2910 settings 

The Pre-Shared Key (above) is the character string that you need to enter in to the ‘Secret’ field of the iPhone L2TP setup (see below). In order to get a connection, the iPhone needs to know the Dial-in user’s Username, the Dial-in User’s Password, and the IPsec Pre-Shared Key.

And the following is the remote user settings

iPhone settings

Go to settings → VPN

The server is the IP address or domain name of the 2910
The Account is the ‘Remote Dial-in User’ Username on the 2910
The Password is the password defined in the ‘Remote Dial-in User’ account name password field on the 2910
The Secret is the ‘Pre-Shared Key’ that was entered on the ‘IPsec General Setup’ on the 2910