Teltonika firmware is based on openwrt. They provide means to setup IP passthrough, so that openwrt acts as a dumb modem and passes its WAN IP to the LAN device. Here is a video about it. Jump to 3:03 for the interesting modes Passthrough and Bridge.
Anybody knows how can it can be done using stock firmware?
Ask the Teltonika people.
Something like these?
https://openwrt.org/docs/guide-user/network/wan/dmz-based-bridge-mode
https://openwrt.org/docs/guide-user/network/wan/bridge-mode
https://openwrt.org/docs/guide-user/network/routing.example.1.bridged.dmz
Teltonika router needs the MAC address of the primary router, so i am guessing it is not this.
What do you want to accomplish?
Without knowing that it is very difficult to offer options to you.
I am not sure what part of "IP Passthrough" is not clear. It is a standard feature of 4G router like Cradlepoint and also on openwrt based devices like mofi.
IP Passthrough means that the 4G router (openwrt device) acts like a bridge device (NAT disabled) and passes the WAN IP to the primary router/firewall (pfsense in my case).
Teltonika has the GPL sources listed for the firmware. This script i believe sets up the Passthrough. It has references to proxy arp.
Another discussion on the same topic is here.
This can be easily manipulated in OpenWrt. Option macaddr
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 WAN 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 wan so that both ports have the same mac address.