Samsung s5 mini - delock cable mouse / keyboard not working
OTG needs to be supported at the kernel level and you device does not appear to have that with stock kernel as you can see from cursory Google search. 

Samsung S5 mini supports Gamepad through OTG? offers a way of flashing a custom kernel to override this limitation pointing to a XDA thread. You would need root to attempt this

Perhaps some videos which claim OTG support have this kernel flashed or custom ROM enabling this.