matricom g-box Amlogic s802

I have had this box for a long time and it never really  worked well.

I would like to install Ubuntu linux on it and then run plex media server.

I have seen guides but when using a 16gb usb stick, nothing seems to work. It just loads into andriod os.

I was only able to get the s812 image but from what I find, people say that it is ok for the S802.

Can someone please help?