Diagnosis
A solid red light means the Alien router itself has no internet connection from your modem. Your devices might still be working on the local network, but the router can't reach the wider web.
How to Fix It
This is almost always a modem-router handshake problem. 1) Unplug the Ethernet cable from the Alien's Internet port (the blue one). 2) Unplug the power from your cable modem or fiber ONT. 3) Wait 60 seconds. 4) Plug your modem back in and wait for all its lights to settle to their normal state (this can take 2-3 minutes). 5) Now plug the Ethernet cable back into the Alien. The Alien's light should cycle and then turn blue within a minute or two.
If That Didn't Work
Try a different Ethernet cable between the modem and the Alien. Also, log into the AmpliFi app, go to Internet, and make sure the 'Connection' type is set correctly (usually 'DHCP' for cable/fiber). If it was set to PPPoE and your ISP doesn't use that, it'll cause a red light.
Heads Up
Some ISPs lock the modem to the first device it sees. By rebooting the modem with the Alien disconnected, then connecting it, you force the modem to recognize the Alien as the new device.