Airport Extreme 7.6.1, problem with connection at 5 GHz

I have a problem with connection 802.11n on 5 GHz after update firmaware to ver. 7.6.1. All my devices, iPad mini, iPhone 4, and MacBook PRO are connected only to 2,4 GHz 802.11g. With the update started also problems with Panasonic tx-p42vt30 connection to the Airport. I get connection on TV after restarting router and waiting a few minutes. But when I turn off the TV and turn it on again the connection is lost; and I have to restart Airport again. I think that there is some problem with DHCP server or with Wifi setting.

Does anybody have any idea?

Thanks,

MacBambus.