Orbi RBR850 with Centurylink Fiber

I have the same setup, and I couldn't connect the Orbi router directly to the ONT (the Optical Network Terminal, the box the fiber connection goes to). You need something in between the ONT and the Orbi to handle VLAN tagging.That could be a modem, but it can also just be a network switch. Just make sure you get one that handles VLAN tagging -- I bought a ProSafe switch and that did the trick.

Here's an article on it: https://www.dslreports.com/forum/r31043915-How-to-almost-use-any-router-with-CenturyLink-s-1-Gig-Internet