First you have to enable the Dynamic DNS via the customer center:
1. Log in to the customer center
2. Select your contract
3. Click on "Dynamisches DNS"
4. Choose under "Dynamisches DNS Domain erstellen:" your desired domain and click "Anlegen"
Now you see a table with 2 columns. In the left column you see your "Dynamisches DNS - Domain" and the corresponding Authhash in the right one.
Setting up the Fritz!Box
Switch on the remote maintenance in your Fritz!Box: (Internet ? Clearances ? Remote Maintenance)
Username: Enter a username you like
Password: Enter a password for your user
Rufen Sie die Einstellungen für Dynamic DNS auf (Internet ? Freigaben ? Dynamic DNS):
Enter following data in the settings for Dynamic DNS:
Dynamic DNS provider : Customized
Append your Authhash after ah=
Domain name: Here you enter the "Dynamic DNS domain" as displayed in the customer center.
The fields for username and password may stay empty since the modifications of the DNS settings are authenticated via Authhash.
Subsequently click on "Apply".
Now it may take up to 10 minutes until the DNS settings are applied.
With a ping command from the command line you can check whether the setup of Dynamic DNS has been successful. If pinging returns the IP address of your Fritz!Box the setup was successful. Provided you enabled the remote maintenance in your Fritz!Box you can reach the web interface via https://<Dynamic DNS domain>.