Hello,
I had this problem before but last firmware update (version that has double tap to wake up) and last Windows 10 Mobile builds (Anniversary Update) resolved this problem.
Before: after some minutes/hours WiFi 5GHz drops and no possibilities to reconnect because no network is found (auto or manual in Settings app). Only a reboot temporary resolve this.
Now: sometimes (rarely) my 5GHz connection drops but it reconnect automatically few seconds after. No reboot or manual action are required.
So, for me, this issue seems resolved with last firmware update and last Windows 10 Mobile updates. Probably bad network management in firmware and in Windows 10 because I had some similar issues in Desktop before Anniversary Update.
Did you install the last updates?
Thank you for the reply .
I have installed the latest firmware and the anniversary update .
from the first day i got the phone it did not see any 5.0 GHz Wi-Fi . After the firmware upgrade and software update nothing was fixed . i even did a hard reset and nothing changes .
now after the update even my VPNs are not working . ( VPN connects but internet traffic is not sent through the VPN ) .
Anyways , I will contact Microsoft tomorrow and see if they can help.
i will report back if any solution worked .