I've encountered an odd issue with the DIR-857 (A1 FW1.03). All my iOS devices (ipad2/3, iphone5, ipod touch4) cannot connect to the App Store. But I can surf with Safari. This occurs on both 2.4Ghz and 5Ghz wireless connections.
I was able to resolve the issue when I turned off 'Short GI' option in the Advanced Wireless section.