Struggling to get a cancelation fee of my name

I received a wifi house modem on the 9th of feb and the modem was not working like they told me over the phone.I send back the modem on the 15th and i was charged with a cancelation fee that i am not supposed to pay because it was send bak in 7 days time.I spoken many times to mnt over the phone and they also told me i dont have to pay that cancelation fee. Its the 19th of July and still not sorted out like they told me over the phone.how much longer must i struggle to get this sorted out.