Sometimes we need point-to-point communications between different machines, but all the machines are using industrial 4G LTE router to access internet. As we all know, 4G LTE router is obtaining dynamic IP address from the ISP, unless you are using special private APN SIM card. So now, build a VPN channel is good choice. We can set all the 4G LTE router as VPN client, and you can build the VPN server in your office or cloud (VPN server requires fixed public IP), then all the VPN clients will connect to the VPN server to form a local network area. At this time, you can visit any machine in the network.