Help! Orbi RBR850 with AT&T BGW-320-500

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

Are you speed testing with a wired PC or laptop? 

What devices are you speed tesitng with? 

So the 10 that the Orbi uses on the LAN side is due to that the modems router is using 192.168.1.1 which is also the default address for NG routers. So NG routers will change to someting else to avoid this conflict, i.e. 10 something if they detect the upstream router has same address on the LAN side. 

10. will work as a LAN side IP address for your home devices. If not, you can change the ISP modems IP address from 192.168.1.1 to 192.168.2.1, then you can change the IP address on the Orbi system back to 192.168.1.1 if thats what you prefer to use. I prefer to use 192.168.0.1 for my needs. 

https://community.netgear.com/t5/Orbi/Can-t-change-IP-address-for-my-RBR850/td-p/2118852/jump-to/first-unread-message