Internet via cable connection without problems

I bought the cable because with us, the socket for the cable Internet is (little space and no outlet near) at an unfavorable spot.
So I once put this cable across the hall to the modem. The length was necessary because it was placed over a door frame.
Despite the information from the provider, you should not use any cables that are longer than 3.5 m, this cable works, at least in our house with 15m length properly and with optimal attenuation values ​​on the modem.