D-Link AC750 User Manual page 188

Section 5 - Connect a Wireless Client
3. Enter the same security key or passphrase that is on your router and
click Connect.
It may take 20-30 seconds to connect to the wireless network. If the
connection fails, please verify that the security settings are correct. The
key or passphrase must be exactly the same as on the wireless router.
D-Link DIR-818LW User Manual 183