DIAGNOSIS A solid red Internet light means the router cannot establish a link with your modem. The Ethernet cable, modem port, or router's WAN port is faulty.
FIX Replace the Ethernet cable between the modem and the router's yellow Internet port. Use a known-good Cat5e or Cat6 cable. Ensure it clicks firmly into both devices. Power cycle both modem and router after the cable swap. The light should turn white after a successful link is established.
IF THAT DIDN'T WORK Test the router's WAN port. Move the modem's Ethernet cable from the router's yellow port to one of its gray LAN ports. If the Internet light goes off and you can get online (temporarily), the router's yellow WAN port has likely failed. This requires warranty service.
HEADS UP Before concluding the router is faulty, connect a computer directly to your modem with the same Ethernet cable. If you still get no internet, the problem is with your modem or ISP service, not the router. Contact your ISP to check for an outage or modem provisioning issue.