Customer owns 2WIRE eqpmnt, makes no eqpmnt changes, 5 yrs later receives monthly eqpmnt rental fees

I have been a U-verse customer since Feb 2012. We purchased the 2WIRE 3600HGV Internet Gateway CSI kit for $100. AT&T has confirmed that we own it (It was part of a special promotion, offer code GOLDSTAR, that also provided free internet installation by an AT&T technician and a special 12-month introductory offer for the monthly U-Verse internet charge; no TV). We have made absolutely no changes to our equipment. Not even temporarily. Not on our own, not through a third-party technician, not through an AT&T technical service call. The equipment still sits working hard on the shelf where the AT&T technician placed it back in 2012. Nothing has changed except our bill, which saw a monthly increase of $7 for equipment rental starting in September of 2016. Nothing happened in August or September of 2016 to cause such an increase; nothing changed. The original equipment still chugs along providing our internet service. The billing code obviously changed, but the equipment did not. I have confirmed these facts with AT&T and answered the same questions many times, (1) through this community forum, (2) through AT&T "Chat Now" boxes, (3) through AT&T private messages, and (4) through numerous phone calls to AT&T customer service going back to October 2016 (oh, to have those hours back). I have invited AT&T to come to my house and look yourself if proof is needed. Still AT&T has not corrected the error. I just got another bill today and the invalid fee is still on there. On Sept 14, "Chris" in the billing dept. said he definitely sees the billing code error on our account (the code was mistakenly changed to the code for an AT&T modem, but it is our modem). He said he would fix the billing code error and correct the past charges, but that he needed 48-72 hours to peruse our past bills since Sep 2016. He promised that AT&T would call my cell phone after the 72-hour delay to explain the findings and corrections. I have heard nothing. AT&T has my address, my cell phone number, and my email address. Nothing. Nothing except of course that yet another incorrect bill came today quite consistently. How many AT&T associates and how many months does it really take to fix a billing code error? This is my third attempt to reach out through AT&T Community Forums, hoping that somebody at "AT&T Cares" and will please have mercy and heeeeeelp. I am weary and now begging.