I'd recommend starting with the steps explained Here to split the 5GHz wireless frequency away from the 2.4GHz band. By default they're 'merged' into a single wireless network name and your devices connect to the best frequency automatically but sometimes it's better to manually connect them up especially Apple devices.
Once you've 'split' them up you should then see two wireless networks in the list of available networks on your iPhone you'd be able to choose from when connecting to the internet.
Let us know how you get on.
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