ᐅ Poor Wi-Fi in a Newly Built Home Despite Fiber Optic Connection

Hello. Today, after 6 months, we finally got internet in our new build. I set up the fiber optic connection on the FritzBox 7590 router. For that, I reset the router to factory settings and configured it according to the fiber installation guide. Then I updated it to the latest version. Somehow, the Wi-Fi isn’t as fast as expected. What could be the reason? The router is located in the utility room (ground floor corner) on a cabinet. Our living room is in the opposite corner on the ground floor. There, we have 3 out of 4 bars, sometimes even less. The speed is not particularly good either.

In the upstairs bathroom (directly above the utility room), the connection is practically non-existent. If I go to the upstairs hallway, I get 3 bars. Shouldn’t the signal actually be stronger throughout, even without a repeater?

The fiber modem and the FritzBox are connected by an Ethernet cable I already had. Should this cable be upgraded? Are there any other tips?

The switch for the LAN outlets isn’t connected yet. I still need to order it and install the wiring.