Solved- N5300 USB port cycling on and off in standby
I have seen similar posts elsewhere, wanted to share what worked for me. I wanted to use a PowerSSR Tail to control cooling fans and dongles behind the set and instead of turning off and staying off, the port would cycle on for 3 minutes or so, off for a minute of so, back on for 3... Under Network Settings I saw an option enabled that said, "Allow Mobile Device to Turn TV On". I disabled that feature which I don't need anyway and voila, one cycle and then it stays off!
No reason that should be so affecting only the USB port and not the screen, probably a bug [Samsung engineers], but it did work so I am happy now.