You are right. It just works in router mode.
In AP mode, to access internet, you have to check the option, ” Allow Guest to Access My Local Network”.
http://www.tp-link.com/en/faq-519.html

I need a WLAN-Router / AP with that feature. Are there any tipps what i can do? Or is there a way to suggest a feature request for the firmware?

You can submit a ticket here.
http://ticket.tp-link.com/index.php?/Tickets/Submit

Hi,

The TP-Link R&D dept. has never understood that their routers can be used without the WAN port, I mean like AP or Wireless Bridge...
Consequently, a lot of things is not working properly in these cases...:eek:

So better buying another WLAN-Router? Seperating the guest network from private network in AP Mode is the only thing im missing. So i have to use dd-wrt/open wrt just because of one single feature? lol I would like to stick with the original Firmware, but i want/need that feature.

Also TP-Link doesn't have any Access Points with ac WLAN and Gbit and detachable antennas for homeusers.