Found a solution: (in Japanese) https://www.tp-link.com/en/support/faq/2921/ X20's config has the WPA3 switch, defaulted to WPA2, hidden in advanced setting seciton.

Hi. I just updated my iOS and iPadOS devices to 14.0 and found that iOS is now noting your connection is with weak WPA2 TKIP so should configure your router to use WPA2 AES or possibly WPA3 (or sort...