I use matvt to emulate a mouse. and you can probably use that to setup keymapper. I don't know of any other semi decent keymappers. let alone free ones.

https://github.com/virresh/matvt

I use "AndroidTv ADB Mouse" to use my phone as mouse and keyboard.

I used buttonsremapper to change netflix/prime/disney buttons behavior. E.g i start another app when tap op Netflix button on my tv remote controller

Any touch based phone app would need some sort of mouse emulation to work on ATV.

https://play.google.com/store/apps/details?id=io.appground.blek -- turns your phone into a mouse/keyboard (works on CCwGTV and leaves the remote setup alone)

https://play.google.com/store/apps/details?id=com.fluxii.android.mousetoggleforfiretv -- can toggle your remote into mouse mode (haven't tested)

But Button Mapper is free and works well for most buttons. You have to buy pro to map all buttons.

https://play.google.com/store/apps/details?id=flar2.homebutton

Buy tvquickactions!