IP passthrough is a feature that similar to half-bridge mode, most common on wire/cable, LTE and 5G modem-routers where true bridging like with pppoe is not possible or cannot be allowed.
The device with the modem onboard is responsible of configuring the connection and and running the modem through the management interfaces (serial/qmi/whatever commands) but all network traffic coming from the modem is redirected to another device through its network ports.
It seems like Teltonika devices are using OpenWrt as a base for their device firmware and they implemented IP passthrough functionality like many modems do.
Their sources can be used as a resource to develop the same feature in OpenWrt
You can get their modified openwrt buildroot system from their wiki https://wiki.teltonika-networks.com/view/Software_Development_Kit
or see it on github
if you search for "pbridge" you find all the code related to this feature (that's how they have called it in the code). https://github.com/sarog/rut9xx/search?p=1&q=pbridge
Happy hacking!