Touch interface setup is for virtuap pads and joysticks.
If you want button that nicely snaps to various resolutions of viewport you need to use UMG.
This also can be done hard way with blueprints and old hud, but adjusting to verious screen resolutions and ratios is pain.
Also when going totally custom way calculating in which are touch event happened is mess.
There is no easy way to just paint where touch areas are, you need to calculate it all manually.
So easiest way really is UMG.