Airtel - Airtel My Wifi speed just hovers around 1 mbps whereas they promised me

I brought a new Airtel Wifi dongle to work from Kollam, Kerala on Jan *th from an Airtel store in Kadappakada Kollam. Airtel sales representative suggested that I will get upward **mbps, or atleast ** mbps. I requested for a prepaid connection, however store guys insisted that the quality of service and speed will be better in postpaid connection, so I accepted and paid ****rs for the device. However after reaching home I found that the speed is hardly *mbps ( I used speed test to measure the speed ). I called the airtel store manager the next day and informed him about the same. He helped me raise a complaint and said I will receive a solution in *-* working days. However days passed and no one from Airtel contacted me or tried to improve the speed. For the past several days my speed hovers around *.* mpbs to * mbps.
Since I am trying to regularly contact the Airtel manager, he is now ignoring my calls. I some how reach him using some other numbers etc.
Now hes offering me help to disconnect the connection. I requested my **** rs which I paid for purchasing the device. Hes saying that there is no policy to return the device ( which is locked with Airtel SIM ). I am requesting Airtel to either refund my amount or help increase the speed in my place ( which is within the Kollam City limits.