DIAGNOSIS A slow, continuous blue blink means the router is stuck in its boot sequence. It's trying to start up its firmware but can't complete the process, often due to a corrupted firmware file or a failed update.
FIX You'll need to use the ASUS recovery tool (Firmware Restoration). 1) Download the tool and the latest firmware file for the RT-AX55 from the ASUS support website onto a computer. 2) Connect that computer to one of the router's blue LAN ports with an Ethernet cable. 3) Set the computer's Ethernet IP to 192.168.1.10 manually. 4) Run the restoration tool, select the firmware file, and with the router OFF, hold its reset button while you power it on. Keep holding until the tool detects it and starts uploading the firmware.
IF THAT DIDN'T WORK The process is finicky. Double-check you have the exact firmware file for the RT-AX55 (not another model). Try a different Ethernet cable and port on the router. If the tool never sees the router, you may need to try recovery method by setting your computer to a static IP in the 192.168.50.x range and using TFTP.
HEADS UP This process can take 5-10 minutes. Do not unplug the router or computer during the firmware upload. The power light will blink rapidly during the transfer, then the router will reboot on its own. Only when it's fully booted with a solid white light should you revert your computer's network settings to automatic (DHCP).