DIAGNOSIS The router is stuck in a boot failure or firmware corruption state. A solid red power LED, as opposed to the normal white, means it failed its initial startup checks and can't load the operating system.
FIX You need to force a firmware recovery using the Asus Recovery Tool. First, download the latest firmware for the GT-AX6000 from the Asus support site to your computer. Unplug the router. Hold down the reset button on the back, plug the power back in, and keep holding reset for about 5 seconds until the power LED starts slowly blinking. Then, run the Asus Recovery Tool on your PC, point it to the firmware file you downloaded, and follow the prompts. It'll push the firmware directly to the router over the network. This usually takes 5-10 minutes.
IF THAT DIDN'T WORK Try using a different Ethernet cable to connect your PC directly to the router's LAN port 1 during the recovery process. Also, temporarily disable any firewall or antivirus software on your PC, as it can sometimes block the recovery tool's communication.
HEADS UP Don't unplug the router during the recovery, even if the lights seem stuck. Let the tool finish completely. After it reboots, you'll be back at the factory default setup screen.