from this link we read:
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
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.
xdsl debug multimode config=t1.413issue2+g992.1_annex_a+g992.3_annex_a+g992.3_annex_m+g992.5_annex_a+g992.5_annex_m