Don't worry that the buttons are broken, developer brother is here :D
Soft Keys 2 - Home Back Button
https://play.google.com/store/apps/details?id=com.dogusumit.softkeys2
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Activate the on-screen keys, enjoy it! Don't need root.
This app uses Accessibility services.
source codes : https://github.com/dogusumit/SoftKeys-HomeBackButton
How to use ?
- open app, enable permissions if asked
- set location, size ...
- click "Settings-Accessibility" button
- find "Soft Keys - Home Back Button" and enable it