How to Setup RNDIS or CDC-EEM (CDC-ECM)-based USB dongle?

I would like to install an RNDIS or CDC-ECM based dongle which is having
a fixed IP address inside the dongle and is offering an fixed IP address
on the USB side. The dongle does also have a small local DNS so that the
IP address does not need to be typed. Now I do want to install the
dongle on Turris Omnia but I am failing. I am able to see the dongle as
eth3 but if I say add interface I do not know what to select. I also
tried to add it to the local lan interface by marking eth3 as included
but still it does not work. Dongle should be on the LAN side, behind the firewall.

Using the same USB dongle on Ubuntu works without any problem.