For those customers who require a Private IP address but do not have a hardware firewall/router, they can download our Sonicwall NetExtender VPN client. We would set up the customer with a username and password. the customer would then enter that when to start up the VPN client.
This will then create a VPN tunnel from the machine with the client to the devices with the SIM cards. As soon as the client is disconnected, the VPN tunnel will be broken. Any machine anywhere that has the VPN client installed will be able to access the devices with private IP addresses.