By default, D-Link devices use WPA2 (AES) and will connect if your client supports it. If your iOS device detects that your router, access point, or range extender is using a mixed security mode (WPA/WPA2), the warning message is displayed even though you are connected using WPA2 (AES) security.

Some models will receive firmware upgrades to allow you to select WPA2 only (or WPA3) instead of using a WPA/WPA2 mixed mode. Note that older clients that do not support WPA2 (AES) will not be able to connect to your WiFi network.