AT&T wants 2 Modems / gateways returned that were never at my house

Yesterday 1/23/19 , I got 2 letters from AT&T stating I need to return 2 modems/gateways with serial numbers ,that were never installed ,or even shipped to address, while on phone and chat  with  AT&T ,agent states IGNORE those letters ,because they were generated AUTOMATICALLY , problem with that is , one letter says my service changed / planned  deactivation date  on 1/15/19  with a return by date of  2/5/19 to avoid a fee  ... 2nd letter arrived same day saying   my service changed / planned  deactivation date  on 1/18/19  with a return by date of  2/8/19 to avoid a fee.. next problem is I never authorized a service change EVER ,nor did I have either of the 2 devices in question installed  or shipped to  my house.. HOW CAN I RETURN THESE 2 DEVICES WHEN I NEVER HAVE HAD POSSESSION OF EITHER DEVICES , THIS HAPPENED WITHIN  THE FIRST 25 DAYS ALONG WITH FREEZING ,PIXILATION AND LOSE OF AUDIO HAPPENING RANDOMLY AND SPORADICALLY  5 SERVICE APPOINTMENT PLUS INSTALL ,WITHIN  19 DAYS,