We have confirmed that the nVidia Shield has a problem with the LTE network. This occasionally causes a problem with the cell connection not re-establishing itself. This problem does not exist on 3G so I recommend to change the setting to use 3G only. There is no downside to this.
To change the tablet to use 3G go to the settings menu, select ... More, Mobile networks, Preferred network type: 3G
My apologies for the inconvenience this causes. If we can be of assistance in adjusting the network settings please let us know by creating a ticket.