Orbi RBRE960 constantly dropping wifi to iPhone?

What is the Mfr and model# of the Internet Service Providers modem/ONT the NG router is connected too? - ZTE ZXHN H268A

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, 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

Be sure your using a good quality LAN cable between the modem and router. CAT6 is recommended. - yes cat 6 connected

What Iphone and iOS are you using? - 15.6.1

What is the size of your home? Sq Ft? -  215 sq meters 
What is the distance between the router and about 10 meters - 30 feet.

Please turn off 1 RBS if you have two deployed. You may have too much wifi signal for that size of home.

https://youtu.be/UR0viMLISz4

What channels are you using? Auto? Try Auto and 48 on 5Ghz. I will try setting manual channel 1 for 2.4Ghz and 40 for 5Ghz and 69 for 6Ghz.

Any Wifi Neighbors near by? If so, how many? - 6 neighbors <What channels are they using? 

Do you have the IoT and separate 6Ghz radios enabled on the RBR? - yes both enabled. <Are you using the IoT broadcast? 

Please DISABLE the separate 6Ghz SSID broadcast.