I've had this personal cell spot device for a few years and I totally forget if I'm supposed to return it when I move. I'm relocating to a city where T-mobile has a strong presence and I could probably still use it.
I think you can keep it, but you need to let T-Mobile know about your address change -- there are some sample messages in this article indicating that the Cellspot expects you to be at a registered address: https://support.t-mobile.com/docs/DOC-21333