Internet often very weak?

Our Wi-Fi works fine, but our Wi-Fi is sometimes really weak. So when I download something on the computer in my room on the upper floor of our house, it is at 500 KB / s, which is really not that much. When I put my computer downstairs right next to the router for a test, I had 5 MB / s (which should be normal for me).

It is about LAN, i.e. Wired Internet.

I have sone Lan plugs in my house that you simply plug into the socket (these can conduct up to 200 MB / s. I only use the Lan plugs on the upper floor, but I have the Lan cable at the bottom of the router straight into the router, but it doesn't make a difference, because I have a LAN plug in the socket at the bottom, as well as when I put the cable straight into the router, but the speed is not the same at the top, because my house is older and I haven't run direct internet cables in my house.

I have already tested a wide variety of LAN cables and can also exclude the cable, as the result was better on my laptop with the same LAN cable.

Lan only really brings something if you put a cable directly from the PC to the router. In that case there's nothing more reliable.

If you have Wi-Fi or Powerline in between, this is no longer Lan and the Lan advantages are gone.

Yes, you got it right, all your LAN cables are ok. Your powerline adapters (you have at least 2 of them) are also ok.

Unfortunately, it is definitely due to your unfavorable power network cabling or interfering devices.
These internet adapters are called dLAN or Powerline.

Powerline can work fine, but it can also work badly (like yours). Solutions that sometimes help are:

Try other wall sockets (do not plug into multiple distribution sockets)
Use longer LAN cables to reach wall outlets that are closer together. LAN cables can be up to 100m long, no problem. So shorten the distance that the powerline signals have to cover through the power grid.
Try to use wall sockets that work on the same current phase. So if possible run over the same fuse of the power box.
If it is not possible to use sockets of the same fuse for structural reasons, there are special phase couplers (for Powerline) that are installed in the fuse box by an electrician (material ~ 30 euro + ~ 30 minutes working time)
Try to disconnect disruptive electrical devices that significantly affect your powerline connection from the power supply. Typically interfering, switching power supply of LED lamps, USB and other chargers or power supplies for small devices (screen, sound systems, …) etc… Find the interferer.

PS:

The gross data rate of the Powerline devices on the packaging is never reached in practice (typically 1/5 of this is achievable). Because this perfect gross data rate is only possible if the adapters are operated directly next to each other, which is a pointless structure in practice.