I am having the same issue as most of you. I have posted this in another thread but there doesnt seem to be many responses there.
I have a iPhone 6 plus with the iOS 8.1 update. I also have a new Netgear router/modem D6300. I have a iPhone 4s, 5s, MacBook Pro, Mac Mini, HTC One (M7), Asus G75SX Laptop, PS3 and Sony Home Theatre System on the network. All these devices connect with no problems on the 2.4GHZ or 5GHZ (The ones that can see both frequencies connect to both).
The iPhone 6 plus will not connect at all. Keeps on saying "incorrect password". I have done the following troubleshooting:
1. Updated from 8.0.2 to 8.1.
2. Rest Network Settings 3 times
3. Restored the phone as New not from backup.
4. Erased all content and settings.
5. Reset the router.
6. Changed the password to 6 digit password.
7. Removed the security on the router.
8. Opened more IP's on the network.
9. Enabled Guest Account.
10. Disabled Access Control.
11. Changed Security from WPA to WPA2 and vice versa.
12. Tried on both bands.
It will connect to my work WIFI, which is probably WPA/WPA2 Enterprise. It will also connect to my HTC One (M7) when doing internet sharing (Tethering). It will also connect to my next door neighbours WIFI, which also has an early model Netgear Modem/Router. I have contacted Apple Tech Support, but the best they could do is ask me to do the above troubleshooting steps which I already did. This is very disappointing, as I just came back from Android to iOS. 😟