I'm using a 551L on Verizon and get decent bandwidth. However, the connection drops after about 8-10 minutes without fail. It happens regardless of signal quality or time of day. This happens while the unit is not moving. The indicator on the modem doesn't show a reboot or anything of that nature. It "seems" like the pi is going to sleep but the connection always re-establishes after a few minutes. It's not rebooting, as I'm able to monitor that.