The SIMs are so that the cameras can communicate via cell signal, has nothing to do with portals or anything like that. For whatever reason, GSM (like AT&T) technology uses SIMs, and CDMA (VZW, Sprint, USCC) doesn't. It may be because GSM is the standard throughout most of the world so using SIM cards means that you can travel with your phone and buy a SIM/plan from a local carrier while you are out of the US. Not sure, though.https://www.pcmag.com/article2/0,2817,2407896,00.asp
I don't know what the current Coverts (AT&T or VZW) do or don't use, but if I had to guess, I'd guess that AT&T uses a SIM and VZW doesn't, unless Covert AT&T has inbuilt communications since Covert is locked in to the portal. With Spartan, you can use GoWireless Auto that transmits to text or email, not to the portal and app.
I have heard that it would be very expensive to develop one communication module that would pass certification tests for all (or the main) carriers. Perhaps that is why "unlocked" phones tend to cost a lot more than "locked" phones. People (in general) are willing to pay more for a phone than for a camera.