newer Apple devices can't connect to 5Ghz wifi network

[V.K.]

I just got a new router (Netgear Nighthawk R8500). It has 3 wifi networks: one 2.4GHz and two 5Ghz.
I'm having a problem with one of the two 5Ghz networks: only my older devices can connect to it, namely iphone 5s and iPad Air. My other 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.

 

[cast128]

Do the 2.4 GHz and 5 GHz networks share the same SSID network name?

 

[V.K.]

I tried various combinations of SSID names - it did not make any difference. In any case, I've solved the issue by exchanging the router. The replacement one has no such issues and all my devices can connect to both 5G networks.
This leads me to believe that my original router had some weird hardware problem. Thank you for trying to help.