The Unlimited AT&T $20 a month was never an ebay deal, it's a post-paid contract plan from AT&T. Since AT&T's supply of the ZTE Mobley hotspot dried up they've limited the $20 Unlimited Connected Car plan to only newer vehicles with OnStar.
Was using the mobley this last week and discovered that at&t flashed a mandatory update to the mobley devices that took away the no time out choice in the admin menu. Now the longest you get is 2 hours "after the car is turned off". So every two hours you have to restart the device.
Mine is setting in the Roadtrek in my backyard and is still turned on. I have an IP (internet protocol) camera attached to it and I can look at the camera on my phone from anywhere (Cell or wifi signal required for phone). I have not had to turn it back on since I turned it on in September on my way back home. YMMV