So I found the solution to the problem by installing the official driver for ubuntu 18.04 (
https://www.displaylink.com/downloads/file?id=1261) and the real problem was hidden here for me
https://support.displaylink.com/know...rticles/641668: I was using Wayland Ubuntu GUI system and it is reported that "Wayland is currently not supported" as one of the limitations. When started another Ubuntu GUI system everything works like a baeuty and I have my 2 monitors!