Will list possible solutions that worked:
1. Reset all app preferences
Go to setting / installed app / more / reset all app preferences.
No restarting device even needed, click wifi and successfully connect.
= work one time, then problem persists.
2. Change device name to new name without special characters. Change SSID name.
= Not working
3. Clear cache in fast boot.
= Not working.
4. Forget network then reconnect. Also select other SSID then reconnect.
= Not working
5. Set SSID as not broadcasting.
= Not working
6. Set MAC address whitelist.
= Not working
7. Restart modem and/or restart cellphone.
After few tries, I found out that no need to restart phone, just turn wifi off then back on again after restarting modem.
= work, but don't solve issue.
8. Set as open network. Without password.
= Works. Combine with disabled SSID broadcasting to temporarily prevent nosy people from leeching metered connection quota.
But doesn't solve the issue of connecting to secured wifi. (Authentication problem while using the correct SSID and password, sometimes connect sometimes refuse to connect for authentication problem).
9.
Keep trying.. Ganbatte!!
Recap:
Failed to authenticate, even if SSID and keypass are correct, saved, and connected-to before.
Can connect everytime if SSID set as open.
Sometimes can connect sometimes cannot, usually after shutting down phone or losing wifi signal after a while.
(Maybe after 24 hours or so?)
Perhaps DHCP IP usage time limit.
Need to ask for new IP due to expiring allocation in router side.
So: try setting static IP. or find out how the phone ask for new DHCP IP (reset config). But deleting SSID and adding back doesn't work?!
Reset DHCP?
