To enable Carddav addressbooks on your iOS device please follow this steps.
To start we need to create a new account. Go to the settings panel
Then open the contacts configuration
Now “Add new account”
Select “Other” as account type
Choose “Add Carddav account”
Insert your assigned server name, your email address, password and choose a name for the account
Attention: Consider to change the name of the Carddav server according to your domain!
Then click Next. The account settings will be proven.
When your credentials are inserted correctly your new account is created.