riroku, That's unfortunate you had problems with your AX86U since it's not the experience most users seem to be having. All four of my AX86Us with the basic settings I used have been running flawlessly 24/7 for months now... never even needing a single reboot since their original setup... except after the normal configuration or firmware updates. But it's good to know Amazon is replacing yours. Please provide updates after you receive your new replacement router.
If you need any setup or configuration advice from other users in this forum don't hesitate to ask. Let the router update it's firmware during initial start up and setup, don't use Smart Connect, use separate SSIDs for 2.4Ghz and 5Ghz, use WPA2 (don't use WPA3 at this time), try to set up your AX86U as similar as possible to your previous router's basic settings... for example use the same previous SSID and password, same LAN starting IP (192.168.1.1 or whatever it was, etc..), don't use AiProtection, leave QoS off or as default, use the regular Asus firmware with most default settings to start (AX enabled and most other default wireless settings should be ok). Keep your settings simple and basic default for your initial setting until you make sure your new router is working properly. You can always make your other personal configuration changes after you confirm everything is working.