Private IP address (RFC 1918)
Updated
A private IPv4 address comes from one of the three ranges reserved by RFC 1918 for internal networks: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. These addresses are not routed on the internet.
In practice
With a private APN, each SIM gets an address from a private range that you choose so that it does not overlap your cloud networks.