Orbi RBR50 will not work with new Spectrum modem

We have had an Orbi RBR50/RBS50 working with a Spectrum modem/router for 2 years.  We've recently had some problems with internet connectivity going down intermittently.   We called Spectrum, who decided the problem was the modem/router, so we exchanged it for new separate modem and router devices.  We reconnect the Orbi directly to the modem and it cannot get an internet signal.

We plugged connected to the modem to a laptop via ethernet and confirmed the modem is working.  We plugged the Spectrum-provided router into the modem, and that also works.  But the Orbi will still not get an internet signal whether plugged directly into the modem or into the router. 

We have talked to Spectrum support, and they had some process of "registering" one router or another on the modem.  (I've never heard of an ISP having to 'register' a device connected to a modem, but this is not my area of expertise).   The Spectrum router did not work properly until they registered it.  They supposedly registered it back to the Orbi, but the Orbi would still not connect. 

I'm assuming the Orbi did not just die between the time we disconnected the old modem/router and connected the new modem.  So either there is some setting in Orbi that needs to change, some "registration" voodoo that Spectrum needs to do, or some fundamental incompatibility between the new modem device and the Orbi.   How can we get the Orbi working again?