Try this:
1. Go to: Settings > WiFi > Long-press on your network > Forget network.
2. Turn off the phone.
3. Unplug the power to the router for at least 30 seconds.
4. Plug in the router and give it plenty of time to fully reboot, at least 3-4 minutes.
5. Turn on the phone and try to connect.
If still no good:
6. Open a browser and get into Setup in the router.
7. Set security encryption to WPA2/AES. Do not use TKIP or AES/TKIP.
8. Give the 2.4 GHz and 5GHz bands different SSIDs.
Still no good?
9. In the phone WiFi settings, turn off Auto select and try setting the band used manually.
Still no joy?
10. Update your router firmware.