@catinzhat 

That would be because Apple devices are only looking at the encryption type and not anything else on the network like a firewall or security suite like our HomeCare to determine the "security" level.  TKIP encyription is not as secure as WPA3 or WPA2 with AES encryption.  Check your router and make sure its encyrption is not set to Auto.  TKIP is ussually selected by default as it is more universally compatible with legacy devices. If Auto is selected then choose AES, save and reboot both the router and extender.