@sve you could try https://github.com/rvaiya/keyd I haven't used it on the FLX1 but I see no reason it wouldn't work. I use it on an oddball aliexpress umpc with arch/Gnome for mouse click left/right since they use keypad buttons (FLX1 uses gnome if I am not mistaken). It also has a handy tool to determine what keys you are pressing to make sure you know what it sees a keypress as

@alaraajavamma By using the GUI and a text editor, I have now config files. But they have no effect. Do you have a working example?

Hey! Sorry for huge delay :S.
It seems that input-remapper is not working anymore - it did while I write that but after latest updates it is not even starting anymore.
Well luckily I found out much simpler and cleaner approach to this:

sudo apt install evremap

Then make create this folder and config file
~/.config/evremap/remap.toml

config file should look something like this:
device_name = "mtk-kpd"
#This is the device for hardware buttons - use "evremap list-devices" in terminal to see available devices

[[remap]]
input = ["KEY_VOLUMEUP"]
output = ["KEY_UP"]
#This remaps volumeup button to default up button - to see available keys use "evremap list-keys" in terminal

To run this just do "evremap remapt ~/.config/evremap/remap.toml" in terminal

This works in general - have not tested everything so expect it might also introduce some funny things (like my example config will remove the assistant-button function because evremap steals the device for it xD)

@alaraajavamma Thanks. Should the evremap command be run as root or not?