Mixing TIVO Hardware

I got a TIVO Roamio installed by my cable company, WAVE broadband. The DVR box was developed in partnership between TIVO and WAVE. The standard TIVO box has been modified by WAVE to allow using "On demand". The MINI from Wave broadband works with the WAVE Roamio, but I cant get the MINI I bought from Best Buy to work. I want to buy the MINI's to avoid monthly charges. TIVO tech support told me it's a compatibility issue, even tho all the equipment is TIVO. Anyone have experience mixing cable provided TIVO Roamio hardware with other TIVO MINI's you purchased outright and it worked?