My internet connection is by A1 (formerly Telekom Austria) and I wanted to use opendns as my custom dns service.
My current firmware version is 4.8.3.DWVV_TAU_5.1.6.
To change the DNS go to the web interface (http://10.0.0.138) and login with the following credentials (root):
Go to this URL: http://10.0.0.138/index.cgi?active_page_str=page_conn_settings_ppp0 and click “Einstellungen” (Settings). Change your DNS as you like and click “Anwenden” (Apply).
There you go.