@Andy123 

With the modem connected to the blue/WAN port of the router, what about the 4th led? Is it off or orange or green？

If it is off, it means the Ethernet cable is unplugged, please check the connection between modem and router.

If it is orange, please follow the below instruction:
1. Unplug the modem’s power cord and unscrew the coaxial cable
2. Connect the modem to the Router’s WAN port and make sure the Router is powered on
3. Plug the power cord for the modem back in, but leave the coaxial cable disconnected.
4. Wait 3-5 minutes and reconnect the coaxial cable.
5. Wait for modem to reconnect and then check for IP and service.

While if it is green, please login to the web page to check whether it obtains the IP address from the modem.
Note: You can go to Advanced-status page to check it; how to login: https://www.tp-link.com/us/faq-712.html

If it still doesn't work, please connect your computer to the cable modem directly and check its IP address, default gateway and DNS.
Note: How to find them: https://www.tp-link.com/us/faq-838.html Please show us a screenshot of the status-details page.

Then please reconnect to the router by WIRED CABLE and login to the web page; Go to advanced-network-internet page to clone the mac address.
Note: Please clone the computer's mac address.

Any updates, please keep me posted.

Have a great day.