Roku Premiere Cannot See Orbi Router

What Firmware is currently loaded?

What is the Mfr and model# of the ISP modem the NG router is connected too?

Under Advanced Tab/Advanced Settings/Wireless Settings uncheck the Broadcast 5Ghz SSID. Apply. 

What kind of SSID name are you using? Try a simple SSID name to see if the new Roku can see it. 

Try setting up the same SSID name on the modem/router as you have on the Orbi temporarily. Then connect the Roku to the modem/router. When it's setup. Turn OFF the wifi on the modem/router and reboot or power cycle the Roku. See if the Roku will connect to the Orbis wifi SSID. 

Modem Combo Units:
This would be a double NAT condition which isn't recommended. 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 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