moved service to a new Apt, ATT mailed powercord and asked me to send back my old wifi/modem

So like the title says, I moved my service to a new Apt. and ATT mailed me some equipment.

I got the package and it was:

1. Power cord
2. Yellow Ethernet cable
3. Instructions 

Then I got an email saying they were sorry to see me go and I have to return the Equipment.

I'm a little confused, If I return my modem I won't be able to be connected to the internet. Was ATT suppose to add a modem/wifi into that box that got that had the powercord and Ethernet cable?

I really don't want to be out of internet, but I also don't want to accumulate late fees. especially if they thing I now have 2 modems now.

Any advice?