The 6300-CX receives a passthrough IP address that is invalid with a /30 subnet, which prevents certain client routers connected to the 6300-CX from utilizing the passthrough connection due to an invalid gateway IP address.

In firmware versions 16.7.49.12 or older, the 6300-CX always uses a /30 subnet in passthrough mode.  However, not all cellular IP addresses are valid with a /30 subnet.  As a result, the client device received a passthrough connection with a gateway IP address that did not match the ranger of the passthrough IP address and subnet.

Firmware versions 16.10.32 or higher resolves the connectivity issues.  The 6300-CX will automatically adjust the subnet if the /30 subnet does not match a valid range for the passthrough IP address.  So as long as your 6300-CX devices are running firmware version 16.10.32 or higher, you should not see any subnet issues while in passthrough mode.

You can use the following instructions to upgrade a 6300-CX or 6300-LX to the new 16.10.32 firmware:

https://accelerated.com/support/6300_CX/users_guide_web/#par-21

Firmware versions 16.10.32 or higher also include extra options in the 6300-CX's configuration to manually set the subnet in passthrough mode, if desired.  See screenshot below for reference.