The most common reason for getting the message "Register Network Failed" is that your new camera does not have a data plan. Each camera must have its own data plan and it must have that data plan before it can be registered.
If you purchased a data plan from Spartan Camera, you should get both a receipt for the data plan and a second email notifying you that your data plan is active. You can also verify by logging on to my.spartancamera.com, Camera Management > Account > Wireless Subscriptions, and check the status for that camera's subscription. (Spartan AT&T SIMs sometimes have enough data to register your camera and send a few photos, but then you'll see the error message. Just buy a data plan.)
If you've added the camera to your cell phone plan, check your account to be sure the that camera is active.
If you have an active data plan for the camera, then check to be sure that the SIM contacts are clean and that the SIM is seated properly in the slot. Also, check that the antenna is snug, but not too snug.