With IP pass through a connected device can be adressed directly with a public static WAN-IP address.
IP pass-through creates a subnet for the attached device (/29) including WAN-IP - and gateway address.
Problem: In order that the used IP's are still reachable in the APN network, a smaller network (== more hosts per network) must be configured:
Example: IP received from your APN: 10.203.1.2/17
Network: 10.203.0.0/17
HostMin: 10.203.0.1
HostMax: 10.203.127.254
Without configured optional WAN network/ WAN netmask the connected device receives:
10.203.1.2/29 gw 10.203.1.3.
The IP's 10.203.1.1 - 10.203.1.6 are no more reachable in the APN network.
Configure a smaller subnet:
WAN network: 10.203.0.0
WAN netmask: 255.255.0.0
Network: 10.203.0.0/16
HostMin: 10.203.0.1
HostMax: 10.203.255.254
The device receives: 10.203.1.2/16 gw 10.203.128.0
The gateway is outside of the APN network but the hosts are still reachable