One day, while modifying the wireless settings for our Cisco wireless router, I discovered a rather stupid problem. It surfaced when I changed the Network Mode for the 5 GHz network from Mixed to Wireless-N Only. This seemed to make sense since we don’t have any devices using Wireless-A. This is in reference to 802.11a in case anyone was wondering if I had actually meant 802.11ac, which my current router does not support.
And that’s the moment when I was disconnected from Wi-Fi and unable to reconnect. Two different Macs (one MacBook Pro and one MacBook Air) were unable to connect. Once again, I resorted to searching and found the solution. It seems, that for whatever unknown reason, when Wireless-A is disabled on my router then all Macs will decide that they require a different feature enabled in order to connect. In this case WMM (Wi-Fi Multimedia).
The reason for this seems more absurd considering that the support doc implies that it must be enabled in the first place but, before changing the Network Mode, those devices connected just fine with it disabled.
Enabling this capability on my router solved the problem. I’m not sure why. It doesn’t seem to be a feature that should be required simply to connect to a wireless router but there’s no question that enabling it resolved the problem. More details are available via the linked support page included below:
Press Any Key 
You saved my life 🙂 such a stupid thing bothered me for days