My jetpack mifi 6620l has been shutting down after a few hours of usage. This started happening once it installed the latest update (Update 5). I purchased another device (again same model) and it was working great until I installed this update.
I made sure that I have configured it to not got to sleep or shut down.
It appears that this is a feature not a bug. I have been running 5 6620Ls 24/7 on remote equipment for the past 1.5 years with no problems until update 5. As soon as update 5 was installed none of the 6620Ls stayed connected for more than 12 hrs. What is interesting is that they would all go down within 5 minutes of each other. This would indicate that Verizon is forcing the 6620Ls off the network to increase capacity and/or force customers to upgrade equipment.