On this router forcing particular ADSL modulations has to be done using its command line interface (CLI). This article details the procedure and the commands needed for forcing ADSL, ADSL2 or ADSL2+ sync's on BT's 21cn (adsl2+).
adsl2+ (21cn) up-to 20mb customers only
Please note that if you are some distance from your exchange in most cases forcing adsl2+ (g.992.5) will not work as the dynamic line management (DLM) will over ride your setting. More information on the 21cn DLM can be found here
Access to the router's command line interface (CLI) is gained by using telnet, typed at a command prompt (on Windows Operating Systems). On Vista you need to enable the telnet feature; on XP it's enabled by default.
To enable telnet go to control panel → programs and features→ turn windows features on and off,then tick the box next to telnet client and click ok. Windows will now spend a few minutes enabling this feature.
Now telnet is enabled on windows open up command prompt (the easy way is to type cmd in start menus search box); now in command prompt type telnet 192.168.1.254 . You will now be asked for a username; enter Administrator (important - use a capital A). Next it asks for a password; if you haven't changed this on the web interface you should leave this blank; if you've changed the password to log into your router then use that.
(If you have a blank password it is strongly suggested you set one up to secure your router. How to do this can is described here)
Now to set the modulation mode you want to use type the following
xdsl debug multimode config=g992.1_annex_a
or, the following for adsl2
xdsl debug multimode config=g992.3_annex_a
or, for adsl2+
xdsl debug multimode config=g992.5_annex_a
Once the disclaimer message shows your router will resync; finish off by typing saveall which saves the settings, use exit to shut the telnet session down
To switch back to multi mode use the following in place of the single mode command (typed on a single line). Don't forget to saveall afterwards.