The "app" may be confused because there are two Netgear systems and it gets connected to the C7800 and says, "Hey, this is not an Orbi!"
The C7800 combines a DOCSIS 3.1 modem (wonderful) with WiFi and router functions. Either the Orbi or the C7800 should be set up as a router, not both. And, most people will disable the WiFi on the C7800 in favor of the Orbi.
Unfortunately, I have yet to find a way to solve this problem with the "app".
The easiest thing to do is disconnect the Orbi from the C7800, turn it on and log in using either a computer with an ethernet cord or using WiFi. Use a web browser and the Orbi web interface, which is probably at IP 192.168.1.1. Since it has not been set up yet, the admin user ID should be "admin" and the password "password". Navigate to the Advanced Tab->Advanced Setup -> Router / AP mode, and set the Orbi to Access Point (AP) mode. The C7800 will handle all of the IP assignments, port forwarding, VPN, etc. and the Orbi will provide a large WiFi network. Most people elect to turn off the C7800 WiFi to keep the radios from interfering with each other.
After setting the Orbi to AP mode, turn it off. Reconnect to the router and power up the Orbi. It will get an IP address from the C7800, which will most likely NOT be 192.168.1.1 One of the management tables on the C7800 will show which IP it gave to the Orbi. Then, you can connect to the Orbi web interface and complete the setup (change WiFi names - I would match exactly the WiFi and password of the C7800 so that all of your devices will connect automatically).
