My internet works but my router won't let any devices connect to it unless you use a cat. 5 cable : \

what should i do?

You would need to get a CAT5 cable, these usually dont cost much, as the manufacturing of these cables is cheaper and more common.

If you mean wireless connectivity, this could be due to your router not supporting wireless, meaning you would have to get a Wireless Access Point

Answermc escher,;

Answertriangular;