ZTE — Service Center is taking ages to replace my wifi dongle

I am using MTS postpaid connection for last few months. I have a rental of Rs. 999/- + vat & service tax. Few weeks ago my wifi internet dongle started giving me trouble so i talked to a representative of MTS. He told me i had to take it to ZTE service center to replace, MTS cant help me with this. I took it to the ZTE service center on 12th May 2015 and since then they are telling me they'll replace my next week but no real help is actually happening. I am very fed up with their service. I use this data card all the time for my work and this is really hampering my business. I am tired listening to their lame excuses that they dont have a wifi data dongle in their stock to replace mine. More than 2 weeks already passed and I have no idea when i'll get back my device