DIAGNOSIS: This is almost always one of two things: an overheating issue, or faulty/overloaded power. The power supply that comes with it can be sensitive.
FIX: Start with the power. 1) Unplug the router's power adapter from both the wall and the router. 2) Check the adapter's specs—it should output 12V and 1.5A. If you have another compatible 12V adapter (center-positive tip), try it. 3) Plug the router directly into a wall outlet, not a power strip or surge protector that might be failing or overloaded. 4) Ensure the router is in a cool, ventilated spot (not on top of the modem or in a sunny window). Let it run and see if the reboots stop.
IF THAT DIDN'T WORK: A firmware bug can cause this. Log into the router if you can between reboots and go to Advanced > System Tools > Firmware Upgrade. Click 'Check' for updates. If there's a newer version, upgrade it. If you're already on the latest, you can try a 'local' upgrade by downloading the firmware file from TP-Link's support site for the AX55 and manually uploading it here.
HEADS UP: If you're using any 'beta' or advanced features like IPTV or a heavy VPN connection, try disabling them temporarily. They can sometimes overload the router's processor and cause a crash. Also, if the router is under warranty and keeps rebooting after trying all this, it might be a hardware defect—contact TP-Link support.