if you were to swap the two 5Ghz if the problem transferred to the other devices ?
I just got a new router (Netgear Nighthawk R8500). It has 3 wifi networks: one 2.4GHz and two 5Ghz.
I'm having a strange problem with one of the two 5Ghz networks (the other two networks work perfectly): only my older devices can connect to it, namely iphone 5s and iPad Air. My newer devices (ipad pro 12.9 inch, iphone 6s, imac 2017, macbook air 2015) can not connect to it at all. They see the network and it shows full strength wifi signal but when trying to connect they fail and show an exclamation mark over the wifi symbol. I'm completely baffled by this. I tried factory resetting the router, tried several of the recent firmware versions but nothing worked.

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