Poor WiFi in the new building despite fiber optics

It's quite fascinating how almost every term from the field is mixed up here. As others have already said, "good WLAN" has absolutely nothing to do with the bandwidth of the internet connection. WLAN bandwidths or bandwidths in general are not measured with a mobile phone to some internet servers, about whose connection and routing you have no information. You do that, for example, with iperf and various end devices (keyword number and characteristics of antennas) at the spots where you have problems, within your own network. For that, you look at how things are in the air. Are there any interferers, are there overlaps, have the neighbors, for example, polluted their place with 25 crappy WLAN thermostats, and so on. THEN you can see what you can do about it. Everything else is wild speculation that, given the ignorance here, ultimately leads at best to unnecessary investments. Added to this is the fact that the construction style with wooden stud walls and tons of drywall is poison for radio waves in that frequency range. If overlaps and cancellations then occur, it's game over. But hey, why lay cables at all? Everything goes over WLAN nowadays! No idea how often I've already read this here.

Just take a look at what's going on in the spectrum, then you can start optimizing.

Please upload a photo of the installation.
To which port did you connect the fiber optic modem?

The modem only has one port (blue). I connected it to "WON (blue)" on the FritzBox.
The yellow one goes to my laptop.

The corner of the installation still needs to be fixed.

I would be interested in the Ping.. At least that allows for a statement about the "fiber optic" connection.

For example, with our connection (FTTH) we have between 2ms and 3ms. Measured via the broadband measurement of the Reg.
Our neighbor with copper wiring has 10 times that. (Despite super vectoring and "you-don't-see-it" great 300Mbit Telekom line)

Nevertheless, the WLAN signal in the house has nothing to do with that.
As already mentioned by some, you should plan enough network sockets for APs in advance, which can be retrofitted if necessary ;)
The reinforced concrete ceilings absorb a lot – as do mirrors, masonry, etc.
Wooden ceilings are still the best.

Attached are the measurement values 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 is halfway up in the room. A cabinet now stands between the router and the fuse box.

I have ordered a 2400 repeater.

Ping is okay.

You lose about half on the DL.

But then use the 2600 as an access point and not as a repeater.

You lose about half during the DL.

What does that mean?

Do I need to change anything with the wiring? I also switched to WAN because of the modem. LAN is therefore deactivated in the router. The laptop only has a WLAN connection despite the network cable between Fritzbox and laptop.