@Kevin_Z Hi ....thks for reply ...solution not so easy ...in Deco P9 you cannot manually switch off the 5GHz frequency ...it seem that Deco network itself decide when to use 2,4 or 5 GHz ....only...

@Kevin_Z Hi Kevin.... i have installed a new wifi mesh network with Deco P9 speed is good all is perfect but i cannot pair my sonoff wifi switch to the new mesh network ....i suppose that the problem...