All cables between a cable modem, a router or switch and to other network connections are so called “straight” cables.
This kind of cable you cannot use between 2-PC’s! If you want to connect PC <-> PC, you need a so called “cross-cable” .
How and what to do:
Cut about 2 inches /5 centimeter from the outside isolation;
Take care not to damage the isolation of the wiring.
Untwist the wires as: 1. Orange/White 2. Orange 3. Green/White 4. Blue 5. Blue/White 6. Green 7. Brown/White 8. Brown
Cut the wires to approx 10 mm and move the wires as indicated in the drawing; check the position before you use the RJ-45 plier!
Do you need a cross cable?
First make on one end a straight connection as above; the other end of the cable has a different color sequence: