The “Settings” icon (three dots in a line) is always in foreground, that is, it is always visible no matter the interface. Nevertheless some devices (phones and tablets alike), mainly but not solely Samsung, for no known reason, don’t show this button or give access to “Settings” via other physical or virtual button on the device.
The first thing to try is to push the Multi-tasking button (the one generally on the left) a few seconds, and see if “Settings” options appear. If the app disappears either the buttons needs extra pressing time or it is not working. In case of the latter proceed to try the same with the rest of the buttons available, just in case.
Anyway there is an option most interfacer have called “Assistant Menu”. To access that option:
- Go to Settings > Accessibility.
- Tap on Dexterity and Interaction.
- Tap on Assistant menu.
- Toggle the Assistant menu switch to On and configure your settings.
That icon allows to see the basic Android options. One of them is “Settings”, so you can try to access, with the app opened, to access it. If it works (it normally does), you can proceed to customize the app to your taste or needs and then disable (if you prefer it) the “Assistant menu” mode.