Figure 1 - Router Mode; Figure 2 - Ip Pass-Through Mode - red lion MobilityPro BT-5600v2 User Manual

The modem manages two connections at the same time, thus acting as a gateway/router:

Cellular WAN connection:
modem can be configured to automatically and autonomously establish a packet data connection to the cellular
carrier and acquire a WAN IP address.

LAN connection:
/ USB ports. In the case of Ethernet and USB, the modem acts as a DHCP server and assigns a private LAN IP
address to the attached device.
The modem then routes packets back and forth between its WAN and LAN connections, and in turn allows the locally
attached device to communicate with computers or devices at a remote location.
1.1.2 IP pass-through mode
In IP pass-through mode, the modem assigns its WAN IP address directly to the attached host, thus becoming a fully
transparent actor in the communication process. The modem remains reachable through its reserved TCP and UDP ports
so that remote administration and configuration is still possible. Note that this mode allows only one IP address to be
assigned to the first connected device via DHCP (This configuration is not recommended for BT-6x21).
1.2 Modem features

Figure 1 - Router mode

This is the Wide Area Network connection to the cellular network/Internet. The
This is the local connection between the modem and any device attached to its Serial / Ethernet

Figure 2 - IP pass-through mode