Interesting, Have you tried using a different browser / PC to factory reset via UI. Maybe an option to load very old firmware (Version 1.10.04_Build460 ) and see if that does the trick. With pushing the factor reset button on back of router, I have found with other ASUS models, this has to be repeated several times to actually get working
I tried Edge (Chromium), Firefox and Chrome. I also tried to flash the oldest version I could found on the Asus page but every setting back to the previous setting instead of the factory default page and the only thing that remain changed is the firmware version.
The reset button at the back did work after tried around 5 times but have to hold it kinda longer than usual but then after setting everything up and click reboot or turn off/on then all the new setting will gone haha, so now I guess I just keep it turn on and re-setting it if it got wiped
It kinda bad since I saw your message about ASUS stop support this model 
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