Sending UDP packets when more than one network interface is up

[SamsonBox]

I have a esp32 with two network interfaces. The first one is the WIFI_AP interface an the second is the WIFI_STA interface. When the esp32 is only connected to my local wlan and no client is connected to the AP, then I can send and receive UPD packages succsessfully via the WIFI_AP interface (e.i. I can send a SNTP package to a NTP server and I receive an correct answer). But as soon as a client connects to the AP (and the IP adresses are configured) the UDP package is send only to the WIFI_AP interface. From the logs I can see, that the package is send via the WIFI_AP interface. So I never get an answer from the NTP server, which is not reachable though that interface. So my question is: What do I need to configure, that the UDP package is allways send to the WIFI_STA interface?