I seem to have found out what was causing the problem in my case: after uninstalling Blokada 5 everything has been working fine, with no more WiFi issues whatsoever.So you may want the check out any app that works through a VPN or something similar, ...
I've experienced the same issue today: first time as soon as the phone was turned on, WiFi was connected but unable to send or receive data. Then again a few minutes ago, unlocked the phone after been idle a couple of hours, WiFi was on but did not w...