I am having this problem with my Note4 too and I haven't had any luck with any of the suggestions thus far (except for a full factory reset which I haven't tried yet). When I connect my phone to my desktop or laptop, I get the message stating "USB device not recognized" in spite of having drivers installed. Kies doesn't recognize it either. I have tried multiple cables and I have the USB debugging enabled. The code *#0808# does nothing on my phone. I found the USB configuration option in the Developer Options part of the settings and changed it to Media Transfer Protocol but that didn't help (and soon afterwards, it changed back to "Charging" on its own). This all seemed to have happened after the last update.
Oh and I cannot save anything to the SD card because my phone quit reading the SD card sometime last year. I tried different SD cards but it didn't matter. My laptop and desktop read them fine. The phone will read the card after re-booting for about a minute and then give a message about no card being found or something. I wanted to root the phone and go back to the operating system that was on it when I got it to see if that would fix the SD card problem as that seemed to happen after the first or second OS update about a year and a half ago. But since I can't get the computer to "see" the phone via USB, my attempt to root it is on hold. Right now, I am transferring some 400 pictures to my laptop via Bluetooth so that I can have them backed up and then I can do a full factory reset. I am hoping that will fix the USB issue and then I can proceed with rooting it and downgrading the OS.