Are you connecting to the router wired or wirelessly?
I advise against your idea of trying to force it to sync slower. Those stats are already a load of garbage so you would (a) make them worse still, and (b) thoroughly confuse the issue for anyone trying to diagnose the problem.
With a downstream attenuation of 45dB on ADSL2+, even with the default noise margin of 6dB you should be connecting between 6 and 8Mbps, not 4506. On 3dB noise margin it should be a bit higher.
What time of day (and day) did you connect to get that speed?
Did you replace or get any new electrical or electronic equipment at the time this started?
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