help setting up xfinity gateway and Orbi router in bridge mode

What Orbi model system do you have? 

What FW is loaded on it?

If you configure bridge mode on the ISP gateway, then the Orbi system needs to be configured for router mode, not AP mode. 

Regarding ISP gateways, If your ISP Modem already has a built in router and wifi. This would be a double NAT (two router) condition which isn't recommended. https://kb.netgear.com/30186/What-is-Double-NAT
https://kb.netgear.com/30187/How-to-fix-issues-with-Double-NAT
Couple of options,
1. Configure the modem for transparent bridge or modem only 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, Keep the modem in router mode, 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, keeping the modem in router mode 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...
https://kb.netgear.com/000061277/Which-features-are-disabled-on-my-Orbi-router-when-it-is-set-to-AP-Mode