Let's verify if you have all the network cables and connections plugged in to the proper Ports. From the Modem, the network cable need to be plugged in to the Router's WAN or Internet Port. Your computer needs to be wired to the LAN Port of your router.
Choose a diff LAN Port to see if there's an issue with the Port that you're using.
your devices if above is confirmed. The step-by-step guide shd help you.
If above suggestions didn't help at all, perhaps it needs Mac cloning
, depending on your ISP. Who is your Provider?