Most of the Z2 owners noticed that since the 23.5.X.X.XXX updates (Android 6.0) the Wifi is working very bad.
My Z2 disconncets from the WLAN Router every 2 minutes and reconnects.
This is happening not because of the bad Wifi Signal( I'am only 2 Meters away from the Router).
It's because of the new bad reworked WLAN Drivers in Android Marshmallow Firmwares.
So i just took the WLAN Driver Files from the old Android 4.4 or 5.0 Firmware (thb i dont know even more), and replaced the new drivers with the old good ones :3 (THE FIX IS IN THE ATTACHMENT)
I did not think that this would work, but i tried it and im impressed, this is working for me !
The Wifi Signal is very stable, even if i go far away from the router the phone dont dissconnects! ^-^
If somebody has this Problem too, just flash this in the recovery and tell here if it worked for you :3
This must working for D6503 and D6502
Sorry for my bad Englisch q///q