Afaik no it is not supported.
I had enough bad experiences with using OpenWrt to run a modem card anyway so I would personally not recommend to use OpenWrt at all if you must run a "dumb" modem card.
I bought Mikrotik Chateau LTE12 to run my LTE connection for example. It has IP passthrough like everyone else. I don't use this feature, my main issue was about the modem card(s) dropping connection and OpenWrt was not reacting to this.
That said, the nearest thing you can do from Luci web interface is set up a DMZ aka all-port-forwarding.
https://ltehacks.com/viewtopic.php?t=49
If you feel like developing this yourself, you can try to adapt the scripts made by Teltonika vendor (that uses OpenWrt as firmware for their devices) to implement this feature IP passthrough in OpenWrt
Or maybe using trelay? It is basically a brainless packet mirror.
trelay relays ethernet packets between two devices (similar to a bridge), but
without any MAC address checks. This makes it possible to bridge client mode
or ad-hoc mode wifi devices to ethernet VLANs, assuming the remote end uses
the same source MAC address as the device that packets are supposed to exit
from.
you bridge the two LAN and usb0 interfaces of the modem device with trelay
And then you spoof the mac with macchanger or whatever other tool (or UCI as said above), on either the modem wan or the router usb0 so that both ports have the same mac address.