Poor WiFi in the new building despite fiber optics

mannoman. just toss all problems into one pot and stir ... shaking my head ...

Approach the problem systematically.

First of all: what did the OP order?
Fiber optic doesn't automatically mean 250MBit/s.
Then: what arrives at the network termination?
If the difference is too big, first address the problem and have the provider solve it!

Then tackle the WLAN issue. There are literally 1000 posts here and on the internet about what can be done. Since the cables for the outlets (I assume the OP means from the patch panel in the utility room to the outlets on the ground floor, upper floor, etc.) are not yet connected, urgently have this done. Then connect a WLAN access point there; if it should also be from AVM, for example, the 2400 repeaters work well as WLAN bridges (access points) (Mesh!)

If the OP doesn’t understand this post: call a professional and have them do it or keep asking here until suitable answers come ;)

The data from the Fritzbox indicates a 50MBit tariff. Not world-class but manageable.

WLAN is a topic of its own. If the Fritzbox is located at one end of the house, it is not surprising that little reaches the other side. Wi-Fi across floors is usually even worse due to reinforced concrete ceilings and underfloor heating. The only solution is WLAN access points. Usually, one per floor is sufficient. It always depends on the floor plan, materials used, neighboring networks, and your own requirements. I hope you have planned network sockets centrally for access points.

There is the free app "FRITZ!App WLAN" from AVN. With it, you can measure the Wi-Fi strength very precisely at different points in the house.

As has already written, a lot is being mixed up here. We have the classic case where someone didn’t think things through during construction or fell into the misconception that a Fritzbox provides fast WiFi for an entire single-family house. Therefore, several access points (wired) and it will work. Please test the 5GHz network directly on the Fritzbox. It should be significantly more performant there.

We have a Magenta M Home contract with a 50000 connection. We have set up the router in the utility room because the electrician recommended it to be directly at the house connection with the router. Unfortunately, we do not have a central network socket in the house. Although we have a network connection in every room, unfortunately not, for example, upstairs in the hallway. I will measure the WLAN strength again tonight. I have already downloaded the FritzBox WLAN app; the data I showed in my initial post comes from that. I am considering setting up a mesh network with the FritzBox 2400 repeater. I will also check the performance that actually reaches us tonight. The router is located in the corner of the utility room where the circuit breaker box is also hanging. Do I also have to stay as far away from it as possible?

I have the first appointment with the electrician at the shell construction soon and therefore always read very interestedly into the internet topics here. But I haven't really understood the problem here yet.

It seems to me that you are using internet and WLAN synonymously?

Topic WLAN:
Is the WLAN too slow for you now, or the internet? Especially with 5Ghz, the reception already drops noticeably after every wall - technically inevitable. But normally 3/5 bars should not be a problem, 2.4Ghz can then be used as a fallback solution. Are both 2.4 and 5Ghz not available throughout the house?
Otherwise, only repeaters or access points help, which are best distributed across the floors via LAN. If your electrician tells you: Fritzbox in the utility room is enough for the whole house, then that's cheap but not optimal.

Topic Internet:
Then basically I did not know at all that you can order such slow fiber optic? The Fritzbox only connects with "DSL 50000" - that is very far from fiber optic speeds like gigabit. Now the only question is, what is available at your place? Do you really have FTTH or was a copper line installed?