Hi, thanks for posting! for the most part, you won't have to worry about compatibility issues in purchasing a modem. All DOCSIS 2.0 modems will work, no problems. While not 100% verified functional, a broad line-up of newer DOCSIS 3.0 modems will do the trick as well. The Motorola 6121 in particular, while not technically supported, is a DOCSIS 3.0 modem that has been known to work on our systems without trouble. Once you pick up a modem, we can register it. Return the other modem to your local office or we can send out a pre-paid box to return it and we can then waive the $5 lease fee as well.