I have a Samsung Galaxy Note 4 (N910a), and the WiFi doesn't work. I think it's because it has an old firmware version, so I tried to update it, but when in recovery and trying to update from the SD card, it says "Update from external storage is disabled." the same thing happens with adb.
Try flashing the stock firmware with Odin. This should work.
Stock firmware download here
Odin flasher download here
Open Odin
Unzip the firmware zip file
Click on APP and browse to where you extracted the firmware zip file
Boot the phone to Download mode (Volume down + Power button)
Connect the phone and click Start.
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