return phone has been lost in post

Hi I returned my phone to yourselves in July within 14 days cancellation.   I am now being chased for money that I do not owe.

I cannot provide the tracking number as it has been a couple months.  And post office cannot provide a copy.  Dpd have said they have never received it.   I want this to stop.  Id mobile nees to cancel contract whi h should have been done when the phone qaa returned in July.  Help me