Poor WiFi in the new building despite fiber optics

Companies that offer building automation including network installation for private houses sometimes have this in their range.

Ok. But as long as many electricians still control double sockets with single cables and suspect the stone next to the front door behind the keystone, where the key is hidden... Rather a bad idea.

Attached are the measurements with ping (measured via WLAN in the browser).
Measured once from the sofa, then in the utility room where the router stands on top of the cabinet directly in the corner of the fuse box, and once measured with the highest values where the router stands halfway up in the room. Now there is a cabinet between the router and the fuse box.

I have ordered a Repeater 2400.

Your values are actually good enough for streaming, surfing, gaming.

How do you actually notice that your WLAN is slow? Just because there are only 3 bars?

Ok. But as long as many electricians still operate double sockets with single cables and suspect the stone next to the front door behind the keystone, under which the key is hidden... Rather a bad idea.

It’s no different than with a heat pump, where you should take the planning into your own hands.

I assume the blue ones are the network cables? Are there really an incredible 4 simplex cables for the whole house? Whoa.... I've never seen fewer, 7x simplex was previously the record low (or none in old houses) But well, everything goes over WLAN anyway ;) Jokes aside, if there really are only 4 cables, terminate them on 2 duplex sockets (as a patch panel substitute) in the utility room. Connect these to the router (yellow ports). Then put the AVM 2400 upstairs (first floor) into a socket. Set it up as a WLAN bridge, patch cable from the AVM 2400 to the LAN socket upstairs. Done.

Unfortunately, I can't beat that record. We have 9 cables. We have a network socket in every room except the hallway and bathroom because we definitely did not say: "WLAN will handle it." While I assumed that WLAN can cover the whole house, accusing me of deliberately skipping sockets is just as wrong as equating WLAN with fiber optic or the performance of the connection ;) It's nice that you now have 20, 30, or more cables, sockets, etc., but it doesn't help me. I assumed that one socket per room is sufficient and I still basically think so. The fact that the double sockets should also have two cables is obviously a problem now. But for that kind of thing, I rely on the electrician. Well, it can't be changed now; the child has fallen into the well. The fact that the WLAN is bad I derive from the fact that it is often very slow and fluctuates. 3 bars are good, also in terms of performance, but it often drops to 2. Regardless of the number of bars, any loading then takes a long time.

Slow is relatively subjective, you can just run the speed test in the browser next time it feels slow and pay attention to what is displayed there. The FRITZ!Box should also be able to show the current network traffic on the status page.