Use G.dmt only mode to improve slow & unstable TPG ADSL2+ internet connection

My internet connection with TPG ADSL2+ (using Dynalink 1046VW modem router) slow down since last Saturday 16/4. It is very slow, loading gmail takes more than a minute. Downloading speed is only 50KB/s maximum.

Read some posts from whirlpool.net.au. It seems uses G.dmt/ADSL1 mode can make the connection stable.

Here is the default DSL settings:

And change to G.dmt mode:

Click Apply button and reset the router.

The download speed is increased 300KB/s. Loading web page is much faster now.

However, I still need to find out why the speed slowed down when connecting with ADSL2+.