I had to use a new network cable. I bought a ready made cable, and as the plugs are already soldered, I had to drill a large 16 mm hole in the concrete wall to allow the plug to pass through the wall, but the cable is 6 mm diameter so the hole is really visible. I made this really simple cap to hide the hole.

All you have to do is print out 2 pieces, then place the cable between the 2 pieces and force a bit through the hole. You can see the result during a test run in my workshop.