Poor WiFi in the new building despite fiber optics

Yes, you're right. A repeater is usually not any good, but it works without LAN.

Do you have LAN single sockets or double sockets?

Yep, it runs smoothly for me: A Fritz.Box in the attic with an access point at the LAN socket on the ground floor as a mesh. All good.

maybe the problem is also with the „Galsfaser“ Something seems to have gotten mixed up ...

maybe the problem is with the "Galsfaser" after all Something seems to have gotten mixed up ...

You mean it's reversed? *LOL*

Repeaters have no place in a new building.

I was also surprised that the Wi-Fi in the attic of our new building is so poor. We also have a FritzBox in the utility room connected to the fiber optic modem with a 50MBIT plan (MagentaZuhause M). I have a LAN socket in every room. If you have that too, it's best to "sacrifice" one socket for an access point.