Guest network problem

Hello community,

i am trying to run my R7000 behind another router. So far, so good.

i followed a guide to set up a guest network and whether the guest wifi works or the normal wifi.

it seems that it is related to dhcp. If it is activated for the normal wifi it works in the normal wifi, but if deactivated the Internet access only works in the guest network. 

The R7000 is plugged in the normal lan with  a fixed IP address.

what am I doing wrong? And can someone link me to the right tutorial of this community please? 

MrBratwurst