"I set it as a Wifi Extender"
Are the SSIDs for your network and the extender exactly the same? (Ie. If you followed the wizard, and I remember correctly, in repeater/extender mode it offers you a network SSID that takes your main network's ID and adds an "-ext" for extended). If they are not, either rerun the wizard setting the SSIDS to the exact name as your main network, or rename it via the extending unit's web management page.