Hello Christian,
You need to configure the hotspot on the virtual wireless interface if you do want to use two SSIDs.
It should be done according to the guide and the linked article on DD-WRT interfaces. You need to unbridge the interface and set the upcoming interface name (ath 0.1 for TP-Link probably) under Services>Hotspot. If it fails to run good, you need to use an older firmware. Like from the end of 2014. (if the current ones are buggy)
Regards,
Steve