Modem rental charge

I noticed a new rental charge on my bill for April. It was for " Wifi internet and telephone modem rental".

I called on 4/8/2017 to a customer service agent. She told me that I was charge because of the wifi internet modem router (dpq3925). But I never ordered the wifi service to begin with. I was told to get an internal/[phone  modem form cox without wifi to avoid the rental charge. I arranged to have a cox technician exchange my dpq3925 to an intenet/phone modem only (dpq3212 was the one the technician installed). But when I call up the customer service on 4/10/2017 two times and was told I will be charged a rental fee until I get my own internet/phone modem.

So the question is : how do I avoid the rental charge? Is the dpq3212 eligible for free usage with the phone service? How does it work?

Thx.

Cox0138