@Edvaard 

When you are setting up your range extender, the RE will have its own DHCP server so that clients like your PC can connect to it and configure the settings. Have you gone into the settings for your RE and configured the settings? For the network discovery to work, your RE will have to use the DHCP server that is operating on your router. The RE will also have a feature for Proxy Mode, that may allow your devices to be seen separately in the client list on your main router.