Okay, the solution to the problem was to change the cable. I was using a Cat5 flat cable, and the speed was poor. But now, with a different “normal” cable, I’m getting 2.5G.
I also changed the cable on my computer, and long and behold, I now have more than 10 Mbps there too.
But thank you all for your help. Sometimes you just don’t see the real problem.