Technician came today and first he checked the “socket” (don’t know if it’s the right word, but you know what I mean) I’m using to connect my router. Signal was bad already there so he checked the socket that comes to my flat. Signal was also bad there so we went to the basement and he quickly found that the issue was caused by a defective splitter (I attach photo below). He replaced it and all signals went green on his device.
Maybe now you can check again if signal values on my router are fine. I will not be able to test the connection during the weekend, but next week I will have a few online meetings so I will be able to notice if issue was fixed. I will write back then.