Network access issue on 5 GHz frequency

I have recently encountered a problem accessing my home wifi network. I have an iMac, MacBook Pro and iPhone all using our home wifi network via a range extender. While the iMac has been connecting successfully without an issues, both the MacBook and iPhone stopped connecting. All 3 devices are using the same Apple ID and share mail, bookmarks, etc., and have been getting along perfectly prieviously. The MacBook opened the usual splash window with the "The Wi-Fi Network "***********_5GEXT" could not be joined. BlahBlahBlah", and the iPhone was saying that the password was incorrect.

Note: there had been no changes to any of the devices prior to this problem - no new apps, changes to System Preferences, etc., and no problems since the last Mac OS update. Also, both computers are mostly in the same room so the issue is unlikely to be location.

I tried every piece of advice I could get but nothing worked up to and including re-installing the system - but nothing fixed the connection issue. Finally, I turned off iCloud in the iPhone, restarted the MacBook into Recovery Mode, erased the Macintosh HD and reinstalled the Mac OS. This solved the problem on both devices, and worked for several days, but has just happened again.

I have been abled to connect the MacBook and iPhone to the 2.4 GHz frequency and get connected, but the 5 GHz frequency simply cannot be accessed.

Note: the iMac and my Roku device have both been using the 5 GHz frequency throughout this drama.

Can anyone suggest the cause? Or even where I might investigate further ... Thanks

Devices:

iMac 27 inch, 2.93 GHz Intel Core i7, macOS High Sierra (10.13.3)

MacBook Pro with Retina display, 2.4 GHz Intel Core i5, 8 GB 1600 MB, macOS High Sierra (10.13.3)

iPhone 8, iOS 11.2.6

Netgear EX6100 AC750 Dual Band WiFi Range Extender