This guide will help you to change the Annex mode on your FRITZ!Box.
Open your web browser (Internet Explorer, Firefox,…) and go to the web interface of your FRITZ!Box. By default, it should be reachable on http://fritz.box or http://192.168.178.1. Log into the device if you are prompted to.
Go to Internet on the left menu and click on DSL Information in a drop-down list.
Then hit Line Settings tab. At the bottom of the page you can define the Annex Settings.