Setup embedded PC to act as router while connected via 4G LTE

I'm an absolute novice and am asking for some general advice on what kind of networking setup i need to include here.

Background:

The buisness I'm working for is adding IoT capabilities to the scientific instrument we develop and manufacture. This involves putting an embedded PC that has a 4G LTE dongle attached inside the instrument and running software on that embedded PC.

In terms of networking, you can think of the instrument as a PC. The embedded PC is getting all required information from the instrument via websockets or ModBus TCP.

The embedded PC has two ethernet ports. One is used to connect the PC to the instrument.

Question(s):

How do i set up the embedded pc to be able to talk directly to the instrument via TCP/IP?

It's almost like the embedded pc needs to act as a router?

The instrument normally has a static IP.

If this is too much/too little information, please let me know so i can edit and know for future reference.