How to bridge/route/tether 2 network cards ?

[RIHTT 0]

Hello.
I have this scenario:

1.
a windowscomputer with two physical network cards.

a) a normal ethernet adapter with RJ45 port
b internal 4G module. with LTE/Simcard . Mobile adapter

This means that the computer normally can run on both or one preferred interface.
mobile connection is a simcard that connects to the LTE infrastructure and nearby available mobile providers.

It can also use wifi/ethernet of course. It seem to be a hot switch-funtion to use the interface with best connection by default.

2. Now I want to do this:

I want to hook up another, external device and plug it into the computers rj45 port.
And use it as a gateway. To achieve internet access and the computer becomes a "router".
the main goal is that the mobile computer can tether its internet connection and forward it to
external devices. I guess also wifi/bluetooth is a last option.

I want to setup the computer, that it always use the mobile network adapter for internet connection.
With DHCP. etc.
(as a preferred interface)
and if possible = ROUTE or share the internet connection
to the other physical network card. Wich means the normal "Ethernet" and its RJ45 port are configured to be
in some kind of a bridge. / or logically as one virtual adapter.

I guess the role and behavior of the rj45 port changes. Or if not possbile the bluetooth or wifi-adapter could do the same

3. And as this was not enough:

If the computer can manage to run both interfaces simultanous with some sort of IP-table.
or ipv4 with alternative adresses. That would be great.
I still need the computer to communicate with the external device on another layer (software)

So the computer can run applications that needs to connect to the external devices with tcp-ip.

example:

c. Application X on computer 1 connect to external device with host adress "192.168.8.99"

d. external device "192.168.8.99" responds, and return package to application X