Poor WiFi in the new building despite fiber optics

Check what is written in the yellow highlighted area of the FritzBox. And then what you ordered. Should not really differ much from each other ...

I have a LAN socket in every room. If you have that too, then it’s best to "sacrifice" one socket for an access point.

You don’t necessarily have to sacrifice a network socket for an access point.
There are also access points that have their own LAN ports, like the Unifi In-Wall, for example. That way, instead of a network socket, you could install the In-Wall at that spot and still have 2 or 4 LAN connections.

Hello. So far, I have expressed myself really contradictorily. At first, it's only about the Wi-Fi for me, since I haven't connected anything via LAN yet. But a brief info: we ordered a VDSL 50 (Magenta M) from Telekom. That was enough for us before. If necessary, I can upgrade to a 100 line for 5 euros more per month.

I have now measured my Wi-Fi directly in front of the box; 1x 2.4 GHz and 1x 5 GHz band. The pictures for that are attached, as well as a picture of the router settings and our sockets.

Something seems to be set wrong on the router. The LAN connection is disabled. I can't find anything to set. So I was not able to measure yet. The FritzBox is connected to the fiber modem via network cable. That is correct for now, right? What is still missing is the connection between sub and switch.

Again about my expectation of the Wi-Fi performance: As a layman, I assumed that fiber optic and a good router can supply our single-family house. In the living room, we have 2 to 3 out of 4 bars. 2 is then no longer optimal. The corners of the utility room and the living room touch each other (however, there are 2 walls in between). I thought that in the living room you would still have at least a steady 3 bars.

Am I right in thinking that I could also connect the router to a LAN socket in the living room once the modem, the sub, and the modem are wired? That way I would get the router out of the corner of the electrical cabinet (interference signals).

I already assumed as a layman that fiber optic and a good router can supply our single-family house.

Once again: fiber optic has nothing to do with WLAN. WLAN has to do with the transmission power of the router, access point, etc. as well as the structural conditions. If you only have a measly 50 connection, even the best router cannot conjure up a higher speed. It can only forward 50. And if there are walls between the router and the receiver, possibly nothing will reach it anymore. Your measurement result shows that nothing is lost on the way, but everything can be forwarded from the measly connection.

Once again: fiber optics have nothing to do with WLAN. WLAN has to do with the transmission power of the router, access point, etc. as well as the structural conditions

I express myself poorly, but I am aware of the difference.

So here in the countryside, we already belong to the upper class with a 50 connection. Maybe we’ll treat ourselves to a 100 one day. But I have always been satisfied with 50 so far, even though only 40 were coming through.

50Mbit is by no means meager! We also have a 50 Mbps connection and that's enough for many things! You can always upgrade if needed. Besides, the DSL/fiber speed has nothing to do with the WLAN bandwidth of the router! It's about the signal strength as far as I interpret it.