Rock Pi 4B+ (1.72) Ethernet Speed

I bought a Rock Pi 4B+ a few days ago (Version 1.72)
When using ethernet (with DietPi), the speeds are soooo slow.

I could connect the board using wifi, to get all the apt packages updated, but when I try to do this on ethernet: Timeouts, disconnects, all sorts of problems… (I couldn’t get APT updated, which is why I switched over to using Wifi)

When I try to to a speedtest with the ethernet, I get a download speed of 0.09 Mbps

Any idea what could be wrong? / How to troubleshoot?

Run an ip -s link and post it here to check for errors.
Have you tried to change cables? Also to change ports on the router or switch?
Where does the RockPi connect? To a switch, to a router? Are there any bridges or anything unsual, like powerlines, involved?