Ubuntu 20.04 wired connection speed limited

Solution 1:

Oh my god after 2 days battling this and trying every solution possible, i finally fixed it.

I was using Linux kernel 5.4.0, which apparently has various ethernet issues. using Ukuu (https://ubunlog.com/en/ukuu-a-tool-to-install-and-update-the-kernel-easily/) I upgraded my kernel to 5.7.13, and this fixed the problem automatically.

I have done everything from every ethtool command to NetworkManager configuration and even reinstalled the realtek driver multiple times, but nothing worked.

TLDR: Upgrade your linux kernel to at least 5.7.13, and the problem will fix itself.