Is your "Hotspot" on the same ring main as your router. I have the BT Mini Hotspot 500 and find that it works very well when on the same ring, but loses a lot of speed when I try to use it upstairs on a different ring.
Also consider what its plugged into. The main wall socket is always the best with these types of devices as many multi electrical adapters have surge protection within them which causes them to either slow down or not work at all.
Hope you get sorted 🙂