WLAN Driver Issue on Note 3

My SM-N900 had issues with call quality and it kept dropping calls frequently but WiFi was working brilliantly (Original ROM was 4.4.2)

At first I mistakenly tried to update but used a SMN9005 ROM but finally I updated to the correct ROM of android 5.0 but till then, my WiFi does not start anymore. I tried installing many ROMS and Modems but no use, I'm sure that WLAN is OK.

My questions are: 1- How can I debug this issue in deep? I used the service menu and it recognizes the MAC address.

2- Where is the driver of WLAN is located in android?