Changing from 2.4 to 5 GHZ

So I noticed that some devices went from 5 GHz (fastest internet you can get) to 2.4 GHz (slower), like my phone. My phone was not as fast. When I checked the internet speed on my phone during the time I had 5 GHz my download speed was like 250 mbps. Then it went to 2.4 GHz and the download speed was around 60mbps. Is there a way to manually change this or does it do it automatically? Because I'm not paying for high speed internet just for 2.4 GHz