Static IP address (fixed IP on a SIM)
Updated
A static IP address stays the same for a given SIM at every connection, instead of being assigned anew each time. It is allocated by the mobile network gateway, from the subscriber profile or a RADIUS server, using the address allocation methods described in 3GPP TS 29.061.
In practice
On a private APN, a static private address is enough for your servers to reach each device. A static public address is needed only if devices must be reachable from the internet.