To get Ubuntu/Linux on Samsung Dex you need to follow these step:
Download Linux Image using this web address
https://www.linuxondex.com
Download Linux on DeX apk file from apk mirror site (make sure you get latest version)
https://www.apkmirror.com/apk/samsung...
Extract the zip file of Linux Image
Open linux on DeX app and add image file of Linux to the container and click on CREATE
Now hopefully you have succeeded in getting Ubuntu/Linux on Samsung Dex.
If you have any questions, please write them in comments below
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