How to direct Internet traffic via 4G modem while connected to a LAN with gateway?

[Bo Berglund]

I am developing a system on a headless Linux Server box, which will in the end
be on a remote location with only a 4G broadband modem as Internet connection
(USB attached).

So I would like to test its functionality when developing on it including having
it direct all Internet traffic towards the 4G modem device rather than the much
faster eth0 device.
So the system is dual homed with both eth0 and usb0 active (the modem attaches
itself as a network device named usb0).

Can this be done and if so how?

I am using SSH to interact with the system using PuTTY on Windows.


-- 
Bo Berglund
Developer in Sweden