I recently had this problem while dualbooting mint and windows: windows was shutting down the LAN hardware somehow.
Had to disable fast startup (in windows, but I recall some Motherboards have also “fast boot” in BIOS as well?) and also in windows’ LAN adapter settings a power saving option.
This is the thread that helped me: https://askubuntu.com/a/879818
A shot in the dark, but hopefully it helps.
Ah, thanks! Didn’t know that. I guess it wont have an effect then and can be ignored.
Edit: fixed up wording.