15dB attenuation would normally get mid 60s to mid 70s depending on noise levels. Your SNR of 5dB suggests that there is a lot of noise being picked up on the line - potentially from cross talk and that is what is pushing you attainable and sync speeds down.
Always worth checking that you are not creating the noise yourself - trying powering everything off in your house including fridge/freezer, boiler, and making sure devices are not in standby by turning off at the sockets. Just have your modem and PC running and monitor te SNR before and after. If that make little difference then there is little else to do.
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