WiFi poor optimisation

Lately, the Wi-Fi speed in some rooms has been fluctuating between 5 and 50 Mbps. On certain devices, a Speedtest shows over 100 Mbps, while at the same time, another device might only get 5 Mbps. I already have one black booster — could I get another one to help rule out weak signal issues? Alternatively, would it be possible to upgrade to the newer hub? The current one is a Hub 4. 

Thanks