Sorry if this is an obvious question, but did any heatsinks or thermal pads come with it, and did you use thermal paste?
I got the other heatsink when I got my NanoPi M4 (V1), the huge one that you could buy with it. It didn't come with the acrylic top then. I found that my NanoPi M4 was overheating like crazy until I thermal pasted the thermal pad in there to improve contact with the main heatsink piece.