What you have done is made the router into a dhcp client only. The dhcp server has been disabled and The network address has been explicitly removed. therefore there is no address until one is assigned by another dhcp server, such as another router.
Do you have another router?
What are you trying to accomplish in general? (What is your goal for this device)?
But I suspect that the OP will not be able to find the device address since it has no address (it was removed as part of the c't guide, and until the OP can reply with the goal of that tutorial, there isn't yet enough info to know why they recommended removing the address)...
Hopefully the OP has another router (ideally) or other system with a DHCP server (it is possible to enable a DHCP server on a Mac, but not the ideal case if another router is available and/or if the OP can use failsafe mode instead).