C3700-100NAS - Cannot Connect w/ Different User/Different State

Good morning Netgear Community: I recently upgraded my NETGEAR C3700-100NAS N600 MODEM/WIFI ROUTER to a new N1900 router and sent the old machine (C3700-100NAS) to another user. This device is physically in LIKE NEW condition and powers fine. It was originally registered by my Internet Service Provider (xFinity) under my account in Pennsylvania, USA. The new user is located in Seattle, USA. Originally, the new user tried to get the machine online via xFinity phone support, which was unsuccessful. Then xFinity technician visit her home and device was able to connect, but only for about 1-2 days. In followup support calls, this user was told the modem was not connecting either because the device was originally used in Pennsylvania and/or because it is still registered in xFinity under former user (me) account. Well, yesterday this user inform me she took modem to xFinity store and had technician transfer the device from my account to hers. Unfortunately, she tell me the C3700-100NAS still does not connect to internet. Can anyone possibly shed some light on what's going on with this device.... if a modem used in one state cannot be successfully connected online in a different state by a different user? Many thanks for your time and input!