Same problem, I was unable to connect my friends iphone 7 with 5 ghz Wifi while other devices were getting connected flawlessly.
Felt so bad that a 70,000 INR + phone was unanle to support 5 ghz wifi. Ridiculous, felt like going to apple forum and crying and bitching the device.
But then I accidentally discovered a website called google dot com that gave me few steps to try to diagnose the problem. I formatted the phone, reset the networks and still no success. I even confirmed the setting on wifi device. Then finally the issue was resolved after I updated the wifi device drivers. All by a simple steps on website reached through google dot com.
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