This is a lie. I am from Meerut and my profiling was changed from my local exchange.
I had the exact same problem. One night I was getting absolutely fine speed of 10mbps, and next morning it was 4mbps. There was no fault in line or cable. SNR was 5 and 6, attainable rate went down to 6mbps...
You can track the status and progress of your grievance online. In my case the case had been allotted to the concerned authority on the same day. They keep pushing the department against which the complaint has been made.
Just open a grievance at CPGRAMS-Home , make sure to include each and every details, including their ignorance about plans and speeds.
They run around like mad when there is pressure from above.(speaking from experience)
Facing same issues on my TD-W8968. Have already reset it many times, still the issue keeps crawling back.
On a number of times it resets itself without me touching the reset button. Have to do initialization of the internet settings again and again.
So maybe this problem is common in TP-Link...