I have a 100mbps (12.5MBps) connection from my ISP. When I use this connection directly on my PC I do get the full bandwidth as advertised. Now I just bought a new router (Archer C20 AC750 ver.5) and after setting it up in quick set up, I got it working flawlessly - or I thought I did. My WiFi speeds are as expected - I get the full 100mbps even from the next room, which is really nice. Problem is, my ethernet speed has decreased dramatically. I get 10-20mbps down and around 2-3mbps up, and the latency is really high. It's quite clear the router is throttling my ethernet speeds, but I wanted to confirm this so I connected the ethernet cable to my laptop and got the same result.

I have seen other people have the same issue before on this forum, and the solution seems to be disabling the NAT feature on the router's online settings page. Problem is, I can't seem to find that setting anywhere. Here's a screenshot of the page for better understanding https://imgur.com/a/OVQvRYG