Seriously, this is so frustrating, I was really hoping all problems with 5GHz would be fixed by this new firmware.
I have the Late 2009 Airport Extreme Simultaneous Dual-Band II, with the new firmware 7.5.1.
I absolutely can't connect to 5Ghz. When in automatic mode, my Mac mini seems to try to connect to 5GHz, then Connection Timeout happens, and quickly it connects to the 2.4GHz.
When creating a separate network name for the 5GHz, it won't connect at all. I tried waiting several minutes, still can't connect. Tried channels, countries, passwords, network names, mac address filtering. Nothing is making a difference, always Connection Timeout.
And this isn't (or at least it doesn't seem like) my Mac mini's problem, because in the previous firmware, I did manage to connect to the 5GHz, however only when using channels 44 and below.
I have no other Macs, so I can't test the router with other hardware, but it really seems to be the Airport's problem.
I hope someone could help me, or maybe I'll just have to wait for 7.5.2.