Issues with an RBR850 (router mode) and one single RBS850 connected wirelessly

Alright, I just completed my cable ISP service visit. He checked the drop at the access box across the street (OK, no problems), then he checked the connection point at my house (OK, no problems), and finally he checked the cable connection at my cable modem inside (OK, full speed available). 

Now I am seriously confused and leaning toward this being a router issue at this point. I have noticed over the last two weeks that my top download speed is also always 90-100 Mbps no matter how I measure it: via 5Ghz wifi-connected device, via direct ethernet-connected device, etc. I have tried every possible way of measuring the speed, and regardless of which device or connection type it always maxes out at 90-100 Mbps. This wouldn't be a problem necessarily except that I pay for 1Gbps down / 100 Mbps up. Also, I get ~100 Mbps upload everywhere via the same testing methodology, so something is restricting only my max download speed.

I did the following today:

• switched the cable between cable modem and RBR850 to Cat8 - no change in behavior, though I haven't seen a network outage so far
• reset my RBR850 to factory settings, onboarded it, and restored previous settings - still limited to ~90-100 Mbps
• connected an old computer via Cat8 cable directly to modem: dl now >400Mbps (computer ethernet card is likely the bottleneck)

I am out of ideas here. Not only is my connection unstable and prone to going dead randomly, but it appears that my download speed is being throttled even though I have not set a bandwidth limit in the router interface.

Interestingly, the last time I ran speed test within the Orbi interface it returned 1 Gbps download. This was prior to the firmware "upgrade". The cable company confirmed that I am capable of this speed, showing me >900 Mbps at the same endpoint today. The router seems to be the problem.

I'm just going to bite the bullet and buy a different router today to see if all these problems are fixed.