Disconnect from the 2.4GHz SSID and connect to the 5GHz in the Settings menu. You may need to set a different SSID in your router first to the 5GHz band to distinguish.
BTW, sometimes even 5GHz Wi-Fi speed and stability is a very unpredictable thing in general, depending from many affecting circumstances, specially for high-bitrate UHD streaming. Hope it will work for you, but if you have problems with it, than the cable is the sure way to go.