Unless I'm missing something, looks like its more-or-less hitting the maximum speed its capable of.
I have no idea where that Max Rate is coming from as with 6.4dB SNR Margin there is no chance it would sync that high.
The one possibility is that ASUS aren't exactly known for having great DSL modems in them, so switching to one of the recommended modems and using the ASUS for just the ethernet side would possibly squeeze a little more out of it.
Have you always used your own router or were you provided with something from your ISP?