I've read sigfox covereage is hit or miss in Europe, hence many of the poor reviews. Coverage in the US should be much better with AT&T network. I suspect they can't use any SIM due to power consumption. It needs to be a low power consumption network (one of the primary advantages of this tracker, not needing a large battery).
Edit: unit just arrived. Love the instant gratification provided for by Amazon Prime with free next day delivery. Unit is only 1 oz (more precisely, 28 grams per my digital scale) and quite small and thin which allows for more options to hide it discreetly. It does fit inside my 31.6 mm seat tube so may be an option for my carbon fiber eMTB (according to Invoxia the seat tube needs to be carbon not aluminum if you want to hide it there). I think the last option pictured below might be the most secure assuming the internal routing holes are large enough for the unit get a GPS lock and transmit. That said I came up with a simple location to hide the unit that is easy to access for charging the battery that won't be obvious at all to a thief, but it doesn't make sense to broadcast where because if everyone does it the unit will be as easily defeated as a rear light tracker, seat post tracker, etc.
After fully charging the device (1 hour), it was extremely easy to set up. Just download the Invoxia GPS app on your phone, create a login (email, password, birthdate), then move close enough to the tracker and it will automatically connect via Bluetooth and allow you to register it, then you can adjust the settings. Note: the app flashed new firmware to the tracker as soon as I registered it. Nice to know the app can update the tracker. You are not going thru some generic app or portal, which provides for a premium user experience and inspires confidence. Not sure if the generic trackers and apps provide for such updates.
Just went on my first ride and as you can see the tracker is not really set up to track your every move. If you are looking for detailed tracking this device is not for you. I wanted something to track my bike if stolen with minimum fuss and it seems like it will do the job. As you can see the device goes to sleep to conserve battery when it is not in motion. You can change the tracker to provide more frequent updates but it will affect battery life. You can also set geofencing zones, where if your bike is moved it will alert you. Proximity radar provides more precise tracking when you are in Bluetooth range of the unit (it will tell you if you are getting closer or further away as you move). Here are some screenshots of the various app screens.