When Wi-Fi is disabled on a phone, the router will push a networking diagnosis screen to help the user resume the Wi-Fi connection, and some third-party phones (such as Xiaomi) falsely determine that the router is not able to provide Wi-Fi access. As a result, such phones will not try reconnecting to the router after Wi-Fi is re-enabled on them. You need to manually enter the Wi-Fi password on your phone to reconnect to the router.