Orbi RBR50 satellite only giving me 1/10th internet speed

Your modem already has a built in router. 

This would be a double NAT 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

i recommend trying option #3 first.

What is the distance between the router and satellite(s)? 30 feet is recommended in between RBR and RBS to begin with depending upon building materials when wirelessly connected. https://kb.netgear.com/000036466/How-far-should-I-place-my-Orbi-satellite-from-my-Orbi-router

You'll not see much speeds over 400-500Mpbs on AC. Depending upon designe and support of the wifi adapter or device. Wired LAN cable speeds should be near 900Mpbs at the RBR. 

ATT Modems:
https://www.f0il.com/how-to-arris-bgw210-700-modem-router-with-netgear-xr500-router/
https://community.netgear.com/t5/Orbi/How-do-I-setup-bridge-mode-in-the-ARRIS-BGW210-700/m-p/1875994/highlight/true#M84982