OpenWRT and 3G/LTE modem - how to set up IP Passthrough?

Question

mobile connection

There are such devices in nature - on the one hand, a USB for a 3G / LTE modem, on the other, a regular ethernet. When connected to the mobile Internet, the device receives an IP address from the cellular operator, but does not keep it for itself, but "forwards" it through itself to the next device connected via ethernet (for example, some Cisco).
The point is to connect exclusively wired routers to the mobile Internet (without wifi or usb), and even with a direct IP on the WAN interface, from which all sorts of VoIP and VPN do not go crazy (as it could be in the case of double NAT and other perversions ).
Examples of such devices are CradlePoint CBA750B, Proxicast PocketPORT 2, Dovado Tiny and others.
I want to make the same device myself based on OpenWRT.
The question is - who has already done this (or similar)?

Answer 1

There are two implementation options - forward USB over IP. On the target device, you will see a USB device - modem. All kinds of adapters can do this ( AnywhereUSB , for example), as well as the latest linux kernels (usbip package). The usbip client part is also available under windows, but it is broken and not compatible with the latest versions of usbip under linux.
Or put both network interfaces (modem and eth) in a bridge.