Connect orbi without a modem

This would be a double NAT condition which isn't recommended if you have router in front of the Orbi router. https://kb.netgear.com/30186/What-is-Double-NAT
Couple of options,
1. Configure the modem for transparent bridge mode. Then use the Orbi router in router mode. You'll need to contact the ISP for help and information in regards to the modem being bridged correctly.
2. If you can't bridge the modem, disable ALL wifi radios on the modem, configure the modems DMZ/ExposedHost or IP Pass-Through for the IP address the Orbi router gets from the modem. Then you can use the Orbi router in Router mode.
3. Or disable all wifi radios on the modem and connect the Orbi router to the modem, configure AP mode on the Orbi router. https://kb.netgear.com/31218/How-do-I-configure-my-Orbi-router-to-act-as-an-access-point and https://www.youtube.com/watch?v=H7LOcJ8GdDo&app=desktop

Try option #2 first, then #3 if #2 doesn't work for you...

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brewer_ashley wrote:
Unable to connect Orbi since is no modem. House is hard wired & existing router is plugged into wall from ethernet.

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You are correct that every model of Orbi (except one) requires an interface to the network, either a cable modem or a fiber box.

If the existing router is connected to the network using an ethernet cable, that would indicate that there is a modem or fiber box at another location that is the interface to the network.

What is the internet service company?

What is the brand/model of the existing router?