Going on the screen-shot you have provided, the GUI puts the actual attenuation against the Margin so, your actual attenuation is 20.5 dB. T he 12.3 is Power. From the Telnet stats your Margin/SNR is 6.3
So, with a 20.5 dB attenuation you might expect around 50Mbps - depending on the noise levels. The 6.3 SNR means that you are getting near optimum and it is possible that there is a lot of noise including cross talk that is pushing your overall speed down and that around 41-42 Mbps is going to be about the best you get unless there are wiring issues in your property that are causing some of the attenuation or noise.
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