This is a Windows driver problem and not a Plex problem. The cause is typically that Windows automatically installs a Microsoft signed driver for the Ethernet (or Wireless) card, and it’s really really rubbish.
My Plex laptop has been suffering the same issue, but I got around to fixing it this week. In my case the problem was that the Laptop had previously been running Windows 7 and had been upgraded to Windows 10. The Broadcom Netlink Extreme gigabit card was using a Microsoft driver, and Windows claimed no better driver was available. I had to download manually a driver from Broadcom and force the computer to accept the MUCH NEWER driver, because even initially the computer still claimed the downloaded driver was not as good as the Windows driver.
The Laptop will now finally stay connected to Ethernet. You will find Googling for this problem - it is incredibly common and affects multiple brands of laptops.
I hope this post helps you to find a solution.
(For what it’s worth, every time Network troubleshooter ran on both the Ethernet or WiFi card, it would claim the problem was that the Gateway was unreachable, and the only fix was to reset the network adaptor)
