Poor WiFi in the new building despite fiber optics

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

VDSL is not fiber optic - by definition. Either you have a fiber optic connection OR VDSL. With Magenta M and 50Mbit/s, I don't believe in a fiber optic connection.

I thought that you would still have at least a steady 3 bars in the living room.

Bars do not have a direct relation to bandwidth, but to signal strength.

Am I correct in thinking that I could also connect the router to a socket in the living room

Yes, then WLAN in the living room should be better. The fewer walls and especially ceilings between WLAN transmitter and receiver, the better. Placing the transmitter in a metal shelf is also not beneficial. I've seen a lot of that..

The screenshot shows that the FRITZ!Box is connected in WAN mode and there is no DSL, so fiber is already appropriate. Telekom also offers a 50 tariff with 10 up and 50 down.

Good morning. We definitely have fiber optic. The speedpipes for it are in place and we also have the modem for it. Fiber optic was also installed.

Below is a picture of what arrives at the ofa. The door of the utility room and living room were open.

: Regarding your post. THE router and the modem are connected by cable. Yesterday I also connected my laptop by cable and wanted to measure. There should have been LAN available, right?

I measured with the FritzBox app.

Unfortunately, the double sockets are not connected with 2 cables.

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

We have the same FritzBox, also with fiber optic (300), and we are also dissatisfied with the Wi-Fi. The connection itself works perfectly, we have wired most of the LAN sockets and they deliver a solid 300 from the sockets.

Wi-Fi is also fast near the FritzBox, but as soon as we walk through the hallway into our living room the connection constantly drops. Mobile devices then constantly show the "No internet!" exclamation mark on the signal icon. We also had the PS4 connected to the network via Wi-Fi at the beginning. When it connected, it worked reasonably well, but sometimes it couldn't get any Wi-Fi signal at all or could only pull data very slowly. The FritzBox is basically 4 meters straight-line distance from the PS4. Unfortunately, there is a solid wall in between, and possibly the heat pump that's exactly in between interferes as well. Or the underfloor heating. God knows what. In the old house, we never had such problems; there, one FritzBox really covered almost the entire house with Wi-Fi.

Anyway, we have now connected the PS4 via LAN cable, and it runs incredibly fast now. Upstairs we have an access point (Unify) connected via LAN socket. It runs reasonably well, but even there I often lose the connection in the bathroom (also only 3 meters line of sight). Either my phone is faulty (I think my husband does not have such major problems), or it’s the house. My husband does occasionally have Wi-Fi problems but not as often as I do. In the basement, I suspect that the FritzBox is simply not very advanced when it comes to Wi-Fi. We will probably buy a second Unify access point soon and just turn off the FritzBox Wi-Fi. It can’t be my phone, since the printer also runs on Wi-Fi and we had extreme problems connecting it initially because the signal is so weak.

Below is a picture showing what arrives at the ofa. The door of the utility room and the living room were open.

Your problem is probably more the signal strength at 67dbm. Apparently, you only have a 50mbit connection to the outside, it does not matter if it's fiber optic or VDSL. The fluctuations in the values of your short measurement also indicate problems with the WLAN signal strength. Only repeaters help with that...