Have a look at the settings for your router, you should have the option to turn off the 2.4ghz signal. But bear in mind which devices in your house connect to WiFi, as sometimes they don't support 5ghz. The other option would be to name your access points differently, eg 'WiFi2G' and 'WiFi5G'. The you can set your phone to always connect to 'WiFi5G'. Hope this helps.
