Just installed KDE Plasma 21.3.7 and only Wi-Fi’s working. I’m using a USB->Ethernet transformer, it was working fine in Linux Mint.
Also, is 21.3.7 version safe for use? 
Hello @mariarttr,
I guess its not a KDE problem. Its more a kernel problem so the kernel you use doesn’t support your ethernet chip. Perhaps you need an extra driver for that.
Did you know which kernel you have used in Linux Mint?
And can you post the output from these commands with connected ethernet-adapter:
lsusb
inxi -Fxxa
Theres nor reason why it shouldn’t work - unless a very exotic chip (although if it works with Mint?) - insert the adapter and run
lsusb
post the output in text format - please - text format