iOS 6 wifi connectivity issues to VDSL modem

After updating the firmware to iOS 6, my wifi home network is not accessible via iPhone 4S iOS 6 . It worked without any issues in the previous iOS versions. The modem is Comtrend VR-3026e. The modem is fine, because it still works with iPhones on version iOS 5.x, laptops and other mobile devices. I have reset the network settings and cleared the passwords, but this didn't help.