Hello again,
here are few things I have tried & their results.
- restart router / iDevice => no change
- renew lease (wifi settings > scroll down & renew lease assuming using dhcp) => no change
- change channels of 5GHz => no change
- change network parameters like using static IP => no change
- reset network settings in iDevice => works !
- forget network & re connect by entering all data (non dhcp settings and passwords , etc) => works !
I suggest you try the forget network option rather than reset all network settings which might make make every network setting to be entered manually.
my usual network settings are WiFi 5GHz, WPA2 security, DNS = router ip (primary), google dns (secondary), and all iDevices running on iOS 8.0.2.
hope this helps.
Regards,