Hi Vasiliy,
Which Ubuntu did you use? The kernel is 3.14 or 4.9?
If you use VIM2_Ubuntu-xenial_Linux-4.9_mate_arm64_EMMC_2018-05-31, you can download the new kernel which enable USB Serial Converter.
Update the kernel:
khadas@Khadas:~$ sudo dpkg -i linux-image-amlogic-s912-linux-4.9_0.3_arm64.deb
khadas@Khadas:~$ sudo reboot
Thanks.