Some apps ignore the keyboard as well. That's not a bug. Google for instance wrote the Youtube app, not Samsung.

That out of the way I checked this link.
http://www.samsung.com/us/video/tvs/UN55H6203AFXZA

It mentions the usual "USB HID" keyboard. Try any PC keyboard if you want to try it then look up the Logitech K400 which is popular with Smart TVs.
Bob