Thank you for your swift reply!
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I'm located in Europe. I just received a message from Support:
I'm sorry the DIR-868L/E (European hardware version) doesn't has this Extender feature.
Maybe a sepcial Singapore hardware version of the DIR-868L has it.So I guess I'll flash the DD-WRT firmware.
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I couldn't flash the router with the DD-WRT unfortunately. I uploaded the firmware, after the restart it had the standard D-Link sofware on it still. I believed it might have to do with the 1.20 firmware. So I've tried to downgrade to 1.07 first and to go from there to DD-WRT. This downgrade didn't work either. After uploading the standard 1.07 D-Link firmware, it restarted and still showed 1.20.
Would it be possible to downgrade in the Recovery mode only? Do you have experience with that?