It looks as though your target noise margin has been set at 15 dB, and this isn't something you have any direct control over, unless you can use the DMT tool to tweak your router. You can ask your ISP to get the noise margin target manually reduced but you may find it hard to persuade them, and you risk making your connection unstable. Short of this, if you monitor your router stats frequently and see that the margin goes up and down through the day, then if it gets well over 15 dB and you reboot when it's at its high point, it should re-sync at a higher speed.