AX4200 Orbi WiFi 6 (RVR750) not serving anywhere near ISP speeds

I've had years of luck with a Nighthawk X6 R7900 and decided to upgrade when I moved.  Installed a CM1000 Docsis 3.1 modem (only) and an Orbi Mesh setup - AX4200 WiFi 6 RBR750.  Setup went well however I noticed that when wired directly to the router, Internet speeds are ~ 100Mb/s.  Speed tests within the Orbi app and management console reflect the 900Mb/s I expect from the ISP - somewhat ruling out an issue with the modem and the modem's connection to the Orbi router.

Since the speedtest speeds across multiple PCs seemed to hover around 100Mb/s, it seemed that something in the middle might be negotiating at a higher speed.  Ruled this out by directly connecting to the router as well as verifying all LAN ports on the router reported 1000M/Full.  

I've performed numerous restarts of the modem and router, a factory reset, and after speaking with Netgear support, lowered the MTU to 1468 and set the Preamble Mode to short - all with no success - ethernet connected PCs continue to cap off at ~ 100Mb/s.  Hardwiring to satellites ironically gives a little better performance however is till 6x slower than my expected bandwidth.

I've tried various Cat5e cables with the same results.  Any thoughts are greatly appreciated.  The system has been boxed for a bit so unfortunately, coming up on the 90 days and wanted to get a resolution / RMA before my time is up.  Thanks much.