A VPN is a private network that uses a public network (usually the Internet) to connect to devices with Private IP addresses from a machine on your LAN or from home. Instead of using a dedicated, real-world connection such as leased line, a VPN uses ‘virtual’ connections routed through the Internet from the company’s private network to the remote device. The VPN tunnel would be used for those who have a private IP address on their SIMs.
Another option for securing your data is a site-to-site VPN tunnel, where two firewalls are used to create a VPN tunnel. This can be achieved using a IPSEC VPN tunnels. A firewall can be configured on the server side, then sending the config parameters to the customer. The customer would then needs to enter the config parameters onto their firewall/router.
This would then enable any machine(s) on a company's LAN to connect to its devices with our m2m SIMs and vice versa. This is an always-on VPN connection. To create a site-to-site tunnel, customers would need to have a hardware firewall/router capable of setting up an IPSEC VPN tunnel.