Slow Wired Speeds RBK50

Having a router in front of the Orbi will cause problems. 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 

I would factory reset the RBR and setup fron scratch. Do not enable Armor, Access Controls, Circle, QoS or Traffic Meter. Speed test with a wired PC using Ooklas installable speed test app. 

Orbi does work for near 900Mpbs however depends on ISP Modem. Gatway routers are NOT recommended. 

https://community.netgear.com/t5/Orbi/Orbi-RBR50-is-not-supporting-900Mb-on-the-WAN-to-LAN-test/m-p/1827295/highlight/true#M76372

You can also try this:

https://community.netgear.com/t5/Orbi-AX/Orbi-AX-6000-and-AT-amp-T-fiber-gateway-issue/m-p/1890125/highlight/true#M2756