This isn't exactly an answer, but I can't list this clearly in a comment.
Process of elimination needs to happen now.
Things to try. In all of these, you're just eliminating possible causes of the problem. So if the problem goes away after you do each of these steps then it can be reliably assumed that whatever you removed from the equation was the cause of the problem:
- Disable the NATing if possible without negatively affecting your internet connection - eliminates the NAT as a cause
- Connect through a different router - eliminates the router as a cause
- Change the source of your internet connection (try it with an LTE device or something) - eliminates your physical DSL line / fibre connection as a cause
- If you can, try to see whether the problem persists on other computers connected to the network. If it doesn't, its most likely something wrong with your ethernet device or else the software on your computer is causing it.