Charged for services never received

I’ve spoken to several people at T-Mobile but cannot get this situation rectified.

1.5 yrs ago purchased Magenta Plan with 2 phones and option for 2 free watches. Within a week realized that 10$ monthly service fee was requited to operate the watches, so we returned them to the store indicating that we did not want them. Without the service they a were of no value to us. 

As we are shopping around before we renew our plan, visited T-Mobile store and rep showed me how to access bill on-line at which point I saw the charges for 4 lines with 2 phone unrecognizable numbers which are associated with the watches. Evidently, we have been charged an additional 20 a month for 17 months, totaling $340.  

Why did I not see this on the bill before you ask?  Well to save $20/month we are required to use auto pay so we never see a bill. I only just realized this error when I visited the store. 

My contention now is that where I purchased the plan, the store representatives are either unskilled or thieves. They took the watches back. They indicated we would not be billed for the service. They did not properly manage the transaction.  

T-Mobile says there is no proof I returned the watches. I argue that I did, but more significantly we don't and never did have service on them which is what the $20/month is for.  Furthermore, the monthly bill advises us that the watches need to be paired up for service, verifying we don't have service. Moreover, T-Mobile has agreed to discontinue charging me moving forward.  What more evidence is needed to confirm that we have been charged for a service never received? 

I would like a refund for the $340 charged for services not received.