Diagnosis
The router failed its boot sequence or can't establish a WAN (internet) connection. A solid red power light means it's stuck in a failed state, not a normal operating mode.
How to Fix It
First, unplug the power adapter from the wall and from the router itself. Wait a full 60 seconds. Plug the adapter back into the router first, then into the wall outlet. Watch the lights—it should cycle through red, then orange, then eventually settle with a solid white power light. This full power cycle clears its memory and forces a fresh boot.
If That Didn't Work
Check the cable from your modem to the router's yellow Internet port. Make sure it's clicked in firmly at both ends. Try a different Ethernet cable if you have one. The red light can also mean it's not detecting the modem signal.
Heads Up
If it goes red again after a few minutes, the issue might be with the firmware. You'll likely need to do a hardware reset (using the reset button) and reconfigure from scratch, which is a pain.