Hello and welcome to the forum 
There is no pause button, only a play/stop button.
If you select a frame in the timeline and press play, it will play from that point regardless of the start and end frame number settings but will loop back to the start frame.
If you press stop, it will stop and go back to the selected frame.
It is confusing, I’d agree.
You can raise a formal bug report with the Wishlist category here:
https://bugs.kde.org/ asking for a pause button, if you want to, but this may take a while to be implemented if it ever is.
You can also contribute to this discussion topic: