DIAGNOSIS A blinking power light during boot usually means it's stuck in a failed firmware update or has corrupted system files. It's in a boot loop.
FIX Try a factory reset. With the router on, find the tiny reset hole on the back. Use a paperclip to press and hold the button for about 10 seconds. The power light will blink faster, then go out and start a fresh boot sequence. Release when it goes out. This wipes all settings, so you'll have to set it up again.
IF THAT DIDN'T WORK The firmware might be completely bricked. You'd need to use TP-Link's official firmware recovery tool (available on their support site for the AX21). This involves connecting your computer via Ethernet, setting a static IP, and using the tool to upload a fresh firmware file while the router is in recovery mode (usually by holding reset while plugging in power).
HEADS UP Always unplug the router from power before pressing the reset button to initiate a recovery. A reset during a partial power cycle can sometimes get it into the right state for the recovery tool. If it's still under warranty and this feels over your head, contact TP-Link support.