Return equipment - never marked as received

We moved back in June to a new apartment and moved our AT&T U-verse and Internet services with us. 

When we did we had an AT&T agent install our new equipment in the new place and were given a new receiver. He then took the equipment with him and we thought we were done with this. 
However, to our surprise on our next bill, we were charged $166.32 because of an unreturned receiver.

When checking with a representative we were promised that they had found the equipment and that they would issue the credit for the next billing cycle. Now 3 cycles later we still have not received our credit for this "unreturned equipment".  We have contacted AT&T after every bill we received without the credit and have been told once we would see the credit within 24hours (which was not the case) and last time were told that it would be on this current bill, and surprise it was not! When we contacted AT&T this time were told that there is nothing they can do as they cannot find the equipment in their system.

Has anyone ever dealt with something like this, and if yes what should we do? $166.32 is quite a bit of money that we want back.

At this point, we are considering switching providers as we feel like we have been lied to on multiple occasions and since this process is extremely frustrating!