@woozle I use my main router as DHCP server. (TP-Link gets its IP address from main router.) And it seems to work properly. I can reach both main router and TP-Link repeater via their IP-Addresses....

I use my TP-Link device as a Repeater. It connects to my main router via WLAN. Also all my end devices can connect to the extended WLAN of my TP-Link. But still no Internet. What could be the problem?