DIAGNOSIS A corrupted firmware file or an interrupted update process has bricked the router. The boot loop is the system failing to load the OS.
FIX You must force a firmware recovery. Download the correct firmware file for the AX21 (Archer AX21) from the TP-Link support website. Unzip it. Connect a PC via Ethernet to a LAN port on the router. Set the PC's Ethernet IP to 192.168.0.100, subnet 255.255.255.0. Power off the router. Press and hold the Reset button on the back. Power on the router while holding Reset. Keep holding for 10-15 seconds until the Power LED blinks rapidly. Release. Open a browser to http://192.168.0.1. Use the firmware recovery page to upload the downloaded .bin file.
IF THAT DIDN'T WORK The recovery page may not load. Try using the official TP-Link TFTP recovery tool, available on their website. The process is similar: set static IP, hold reset, power on, use the tool to push the firmware to 192.168.0.1.
HEADS UP Always verify the firmware file is exactly for your hardware version (V1.0, V1.1, etc.), listed on the router's label. Never interrupt power during an update. The recovery process requires precise timing on the reset button. If recovery fails, the hardware is likely permanently damaged.